Audio Coreaudio

Audio CoreAudio

Context: $ARGUMENTS

Quick start

• List input/output devices: → Step 1 (macaudio.py list-devices)
• Change default output: → Step 2 (macaudio.py set-default)
• Inspect an audio file: → Step 3 (macaudio.py info)
• Play a file: → Step 3 (macaudio.py play)
• Convert format/rate/bits: → Step 4 (macaudio.py convert)
• Text-to-speech: → Step 5 (macaudio.py tts)
• Build an Aggregate Device: → Step 6 (macaudio.py aggregate-create)
• List HAL virtual drivers: → Step 7 (macaudio.py hal-plugins)
• Install BlackHole virtual cable: → Step 8 (macaudio.py blackhole-install)

When to use

• User is on macOS and needs terminal-driven audio control.
• Wants to change the default input/output/system device without clicking through System Settings → Sound.
• Wants to convert audio files with Apple's built-in afconvert (more format coverage than SoX for Apple formats; handles CAF, M4A, ALAC natively).
• Wants to build an Aggregate Device (multi-input) or Multi-Output Device (mirror output to several devices).
• Wants to install / evaluate virtual audio drivers (BlackHole, Loopback, Background Music).

Not for: Linux (use audio-pipewire), Windows (use audio-wasapi).

The script exits 2 with a helpful message if run on non-macOS.

Step 1 — List devices

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py list-devices
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py list-devices --format json

SwitchAudioSource -a underneath. Each line is <name> (<type>) where type is input / output. If SwitchAudioSource isn't installed the script tells you to run brew install switchaudio-osx.

Step 2 — Set default input/output/system device

macOS distinguishes three defaults: output (music/TV audio), input (mic), system (alert sounds, which can be a different device).

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "MacBook Pro Speakers"
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "Scarlett Solo" --type input
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "MacBook Pro Speakers" --type system

Device name must match exactly (spaces matter, quoting required). Use list-devices first.

Step 3 — Inspect and play files (afinfo / afplay)

afinfo + afplay ship with macOS. No install needed.

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py info track.caf
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py play track.caf
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py play track.caf --volume 128   # 0-255

afinfo reads every Apple format (CAF, M4A/AAC/ALAC, AIFF, WAV, MP3) plus anything the system's registered AudioToolbox components understand.

Step 4 — Convert with afconvert

afconvert is Apple's native format converter. Preferred over SoX for Apple-specific flows (CAF, ALAC, AAC for M4A/M4R, lossless 32-bit float).

# WAV -> Apple Lossless in an M4A
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input in.wav --output out.m4a --format m4af

# 24-bit 96k WAV -> 16-bit 48k WAV
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input hi.wav --output lo.wav --format WAVE \
  --rate 48000 --bit-depth 16

# Stereo -> 5.1 upmix (container must support it — CAF does)
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input stereo.wav --output surround.caf --format caff \
  --channel-layout 5.1

FourCC codes the --format flag accepts: caff (CAF), m4af (MPEG-4 audio), WAVE, AIFF, AIFC. Full list: afconvert -hf.

Step 5 — say + afplay TTS

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "Build complete"
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "hola" --voice Monica
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "saved" --output voicemail.aiff

say -v '?' lists installed voices. Voices beyond the defaults (Alex, Samantha, Daniel) require a one-time download via System Settings → Accessibility → Spoken Content → System Voice → Customize.

Step 6 — Aggregate Device / Multi-Output Device

macOS lets you combine physical devices two ways:

• Aggregate Device — sum inputs and outputs of several devices into one logical interface. DAWs see it as a single multi-channel device. Needs drift correction when the member devices don't share a word clock.
• Multi-Output Device — mirror playback to several outputs (e.g. speakers + BlackHole + AirPods). Output-only.

This wrapper opens Audio MIDI Setup and prints the clickable steps, because programmatic creation needs AudioHardwareCreateAggregateDevice from CoreAudio/AudioHardware.h (Swift / Objective-C). Use when wiring for automation:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py aggregate-create \
  --name "Studio Aggregate" \
  --devices "AppleHDAEngineOutput:1B,0,0,1:0" "BlackHole2ch_UID" \
  --drift-correct "BlackHole2ch_UID"

Member device UIDs come from list-devices (pass --format json for the full UID strings).

For a truly automatable path, write the Swift snippet that calls AudioHardwareCreateAggregateDevice — see references/virtual-drivers.md.

Step 7 — List HAL plug-ins

HAL plug-ins are .driver bundles that register virtual audio devices:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py hal-plugins

Typical output includes BlackHole2ch.driver, BackgroundMusic.driver, Loopback.driver, Aggregate/Multi-Output synthesizer drivers from Apple.

Global path: /Library/Audio/Plug-Ins/HAL/*.driver. User path: ~/Library/Audio/Plug-Ins/HAL/*.driver (rare; per-user installs).

After installing or removing a HAL plug-in, coreaudiod has to rescan:

sudo pkill coreaudiod     # wrapper intentionally does NOT run this

Step 8 — BlackHole (virtual cable)

BlackHole is the go-to free open-source virtual driver — works on Apple Silicon (DriverKit / AudioDriverKit) and Intel, 2ch / 16ch / 64ch variants.

The wrapper prints install instructions rather than installing automatically:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py blackhole-install --variant 2ch
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py blackhole-install --variant 16ch

Runs the install only if the user copies the printed brew install --cask line. See references/virtual-drivers.md for the full BlackHole vs Loopback vs Background Music comparison.

Gotchas

• Soundflower is abandoned. It does not load on macOS 11+ / Apple Silicon. Never recommend it. Use BlackHole instead.
• SwitchAudioSource device names must match exactly. Trailing spaces, capitalization, and the full name (including brand prefix) all matter. Quote-wrap everything. list-devices --format json gives you a copy-pasteable UID.
• There are two "audiodevice" CLIs in the wild. An npm package (node-audiodevice) and a Go binary. Both predate SwitchAudioSource and have different flags. Always specify the package/binary when pointing at them; default to SwitchAudioSource in docs.
• BlackHole, Loopback, and Background Music all use DriverKit/AudioDriverKit on Apple Silicon. Legacy kext audio drivers (Soundflower, old Loopback versions) won't load. If a driver supposedly installs but list-devices doesn't show it, SIP or kernel-extension approval is likely blocking it (System Settings → Privacy & Security → Allow).
• Aggregate Device needs drift correction when members don't share a clock. Without drift correction, multi-USB-device aggregates will desync by seconds over a long session.
• Multi-Output Device cannot be a system alert device. macOS rejects it — system sounds require a real output endpoint.
• afconvert fourCC codes are case-sensitive. WAVE with -f, not wave. Likewise AIFF/AIFC/caff/m4af.
• After installing a HAL plug-in, new processes pick it up; existing ones don't. Apps that cached AudioDeviceID values will miss it until restart. System Settings → Sound usually re-enumerates immediately.
• say runs through the system output by default, not whatever set-default --type input was last set to. If you need say → file, pipe through --output (AIFF) or afconvert afterward for M4A/MP3.
• AppleScript via osascript can hit Sound preferences via UI automation, but it's fragile across macOS versions and requires accessibility permissions. Prefer SwitchAudioSource for scripted default changes.
• CoreAudio HAL IDs are not stable across reboots. Use the device UID (string) instead of the numeric AudioDeviceID when persisting references in scripts / plist / DAW configs.
• BlackHole per-channel variants are not upgradeable in place. Uninstall blackhole-2ch before installing blackhole-16ch, or both drivers show up simultaneously (which is fine — they are different devices, separately named).

Examples

Example 1 — "Swap default output to my headphones"

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py list-devices     # find exact name
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "Sony WH-1000XM5"

Example 2 — "Capture system audio into OBS"

1. Install BlackHole 2ch:

   uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py blackhole-install --variant 2ch
   # copy the printed brew command and run it
   

2. In Audio MIDI Setup build a Multi-Output Device combining your speakers + BlackHole; set it as system output.

3. Point OBS's "Audio Input Capture" at BlackHole.

Example 3 — "Convert a WAV archive to Apple Lossless"

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input master.wav --output master.m4a --format m4af

Example 4 — "Read EXIF from an AIFF"

afinfo covers it:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py info recording.aiff

Example 5 — "Speak build results to me"

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "Tests passed, zero failures" --voice Samantha

Troubleshooting

SwitchAudioSource: command not found

Cause: Not installed. Fix: brew install switchaudio-osx. The wrapper points at this in its error output.

Default device "silently" reverts after plugging in USB interface

Cause: macOS default-reassign behavior when "When playing sound through" is set to "whatever device is connected". Overrides your scripted default. Fix: System Settings → Sound → Output → uncheck "Automatically switch to newly connected devices" (macOS 14+), or persist the default in a LaunchAgent that re-runs set-default on device change.

afconvert: Error -50 (invalid parameter)

Cause: Requested an output format/rate/bit-depth combo that the codec rejects (e.g. ALAC in WAV container, 32-bit float into M4A). Fix: Drop mismatched flags. ALAC belongs in m4af only; bit-depth is AAC-ignored.

BlackHole doesn't appear after install

Cause: System Extension blocked by macOS. Fix: System Settings → Privacy & Security → scroll to the "System software was blocked" banner → Allow. Reboot. Retry.

afplay works but no sound

Cause: afplay uses the current system default output. If you set a weird output recently (aggregate without speakers as a member), nothing audible is reached. Fix: Re-run set-default with a known-good physical output and retry.

Aggregate Device session drifts

Cause: No drift correction, or drift master isn't the most-stable clock. Fix: In Audio MIDI Setup, check "Drift Correction" on every non-master member. Put the interface with the best clock (Thunderbolt → PCIe card → USB 2) as master.

Reference docs

• BlackHole vs Loopback vs Background Music comparison, plus the Swift snippet for programmatic Aggregate Device creation → references/virtual-drivers.md.