feat: Audio Unit plugin support for per-app and per-device effects

[iscle]

Summary

This PR implements support for Apple's Audio Unit plugins. They can be toggled per-device and per-app basis. Changes persist across reboots, and custom UIs can be opened. The functionality overall is really similar to other competitor paid audio software for macOS ;)

Screenshots

Features

• Add support for third-party and system Audio Unit effect plugins, applied per-app or per-device (similar to SoundSource)
• Plugin discovery from /Library/Audio/Plug-Ins/Components and ~/Library/Audio/Plug-Ins/Components with live detection of newly installed plugins via kAudioComponentRegistrationsChangedNotification
• RT-safe AU hosting with non-interleaved stereo rendering, pre-allocated deinterleave buffers, monotonic sample time tracking, and heap-allocated AudioBufferList
• Custom Cocoa AU views loaded via kAudioUnitProperty_CocoaUI with IMP-based invocation; falls back to AUGenericView when unavailable
• Right-click context menu on effects to switch between custom and generic interface
• Hierarchical plugin picker with search, favorites (with namespaced ForEach IDs for correct SwiftUI identity), and crash history warnings
• Effect chain view with per-effect enable/disable, global bypass toggle, factory preset selection, and remove
• Failed plugin instantiations tracked and shown with warning icon
• AU parameter state persisted on plugin window close and app quit via ClassInfo plist
• Bypass state persisted across app restarts
• Favorites and crash history stored as observable AudioEngine state
• Crash guard integration: tracks active plugins via FNV-1a hashes in async-signal-safe buffers, writes crash file via POSIX on signal, auto-disables offending plugins on next launch
• Device AU chains reload correctly on device switch (crossfade, fallback, multi-device)
• Tail time tracking: continues rendering through AU chain after input goes silent (reverb/delay tails fade naturally)
• Bypass respects tail time (skips tail rendering when bypassed)
• New chain rebuilds preserve bypass state
• AUChainState model consolidates entries, bypass, and failedEntryIDs per scope
• AudioEngine owns all AU observable state (appAU/deviceAU dictionaries); SettingsManager handles persistence only
• Popover favorites use @State with namespaced IDs (NSPanel observation boundary prevents @Observable tracking)
• 45 new unit tests covering descriptor codable, scanner discovery, AU instantiation, RT rendering (lowpass attenuation + reverb tail), bypass passthrough, settings persistence, and crash history

Signal flow

Volume ramp → EQ → AutoEQ → [Per-App AU Chain] → [Per-Device AU Chain] → Loudness EQ → Loudness Comp → SoftLimiter

Test plan

• Build succeeds with zero errors
• All 857 existing + 45 new unit tests pass (0 failures)
• AU rendering verified: AULowPassFilter attenuates 10kHz signal by >40dB at 200Hz cutoff
• AUReverb2 impulse response produces reverb tail with 100% wet mix
• Bypass toggle passes audio through unmodified
• Disabled host passes audio through unchanged
• Settings persistence round-trip (app chains, device chains, favorites, crash history, bypass state)
• Factory preset selection updates UI and persists
• AU parameter changes saved on window close and app quit
• Favorite star updates immediately in popover
• Custom Cocoa view loads for AUs that support it; generic fallback works
• Manual: switch output device with AU effects active, verify no glitches and device chain reloads
• Manual: add effect to device, verify all apps routed to it get the effect applied

🤖 Generated with Claude Code

[evkaw]

Is it possible to add the option to add the FX onto any app instead like in SoundSource instead of assigning it to a sound device? Thank you.

[nifhanif]

Please consider including this in the next release.

[mrbeats09]

+1 on that, still waiting on this feature a month later