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Audio Mixer

Per-app volume control for macOS. A native menu bar app using Core Audio Process Taps to independently control audio levels for every running application.

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Audio Mixer — Per-App Volume Control for macOS

A native macOS menu bar app that does what Apple doesn't — individual volume sliders for every application.

Audio Mixer

The Problem

macOS has no built-in per-app volume control. Unlike Windows' Volume Mixer, there's no way to turn down Brave while keeping Music at full volume. Commercial solutions exist (SoundSource — $39) but use legacy kernel extensions or proprietary HAL plugins.

The Solution

Audio Mixer uses Apple's Core Audio Process Tap API (macOS 14.2+) — the same low-level system that powers screen recording audio capture — to intercept, mute, and re-route individual application audio streams with real-time gain control.

How It Works

  1. Process Discovery — Queries Core Audio's process object list and walks the process tree (via sysctl) to group helper processes (like browser audio sandboxes) under their parent app
  2. Audio Interception — Creates a CATapDescription per process with muteBehavior = .mutedWhenTapped, silencing the original audio in the system mix
  3. Aggregate Device — Builds a private aggregate audio device that combines the tap input with the default output, using the tap's UUID for routing
  4. IO Proc Callback — Registers a real-time audio callback via AudioDeviceCreateIOProcIDWithBlock that copies tapped samples from input to output, multiplying by the per-app gain factor
  5. Zero-latency — All gain is applied in the audio thread callback — no buffering, no AVAudioEngine, no AudioUnit overhead

Technical Challenges

  • Tap UID discovery — Process taps don't expose a kAudioDevicePropertyDeviceUID. The aggregate device references taps by the UUID assigned to CATapDescription, not by a HAL property query
  • Browser audio sandboxing — Chromium-based browsers (Brave, Chrome) route audio through sandboxed helper processes (audio.mojom.AudioService) that have no NSRunningApplication entry. Solved by walking the process tree via sysctl KERN_PROC to find the parent app
  • Perceptual volume scaling — Human hearing is logarithmic; linear gain multiplication sounds wrong. Applied perceptual curve mapping

Technical Stack

  • Language: Swift 5, SwiftUI
  • Audio: Core Audio HAL API (AudioHardwareCreateProcessTap, AudioDeviceCreateIOProcIDWithBlock, AudioHardwareCreateAggregateDevice)
  • UI: SwiftUI MenuBarExtra with .window style
  • Process Inspection: sysctl KERN_PROC for parent process tree walking
  • Target: macOS 14.2+ (Sonoma)

Features

  • Auto-discovers all audio-producing applications
  • Per-app volume sliders (0–100) with real-time control
  • Per-app mute and global mute-all
  • Master volume control
  • Groups browser helper processes under parent app
  • Volume preferences persisted per bundle ID
  • Menu bar-only app (no Dock icon)