MixBar

A macOS menu bar app that works like the Windows volume mixer:

• Output switcher – pick which connected output device (speakers, AirPods, HDMI, USB…) is the default, and set its master volume / mute.
• Per-app volume – every app that uses audio gets its own slider and mute button. Levels are shown for apps you have adjusted.
• Media controls – previous / play-pause / next buttons that drive whichever player is active, plus the current track from Spotify or Music when one is running.

Requires macOS 14.2 or newer (uses Core Audio process taps).

Opening it

Click the three-sliders icon in the menu bar, or press ⌃⌥M (Control + Option + M) from anywhere. If the menu bar is full and the icon is hidden under the notch, the hotkey (or relaunching the app) shows the mixer as a floating panel at the top-right of the screen instead.

Diagnostics are written to ~/Library/Logs/MixBar.log.

Build and run

./build.sh            # release build → build/MixBar.app
open "build/MixBar.app"

./build.sh debug builds a debug binary instead. The app is ad-hoc signed, so it runs locally without a developer account.

Permissions

Feature	Permission	When it is asked
Per-app volume	System Audio Recording (Privacy & Security › Screen & System Audio Recording)	The first time you move an app slider away from 100% or mute an app
Media buttons	Accessibility	Click Enable media keys in the popover footer
Track name	Automation for Spotify / Music	The first time the popover opens while one of them is running

How per-app volume works

Core Audio does not expose a volume control per process, so the app uses a process tap (CATapDescription with mutedWhenTapped). Tapping an app silences its direct output; the app's audio is then replayed into the current output device through a private aggregate device with the chosen gain applied. Apps left at 100% are not tapped at all, so they keep playing straight to the device with no added latency. Volumes are remembered per bundle identifier.

Icon

App/Icon/make_icon.swift draws the icon with AppKit; iconutil turns the iconset into AppIcon.icns:

cd App/Icon && swift make_icon.swift . && iconutil -c icns AppIcon.iconset -o AppIcon.icns

Layout

Sources/MixBar/
  main.swift                 app entry
  App/AppDelegate.swift      status item + popover, MIXBAR_SNAPSHOT debug hook
  Audio/CoreAudioHelpers     typed wrappers for AudioObject properties and listeners
  Audio/OutputDeviceManager  output devices, default device, master volume/mute
  Audio/ProcessAudioManager  process objects → apps (groups helpers under their owning app)
  Audio/ProcessTap           tap + aggregate device + IO proc with gain
  Audio/MixerModel           per-app state, persistence, tap lifecycle
  Media/MediaKeys            play/pause/next/previous key events
  Media/NowPlaying           Spotify / Music track polling
  Views/MixerView            SwiftUI popover UI