Route macOS volume keys to the external monitor selected as the default audio output.
Monitor Volume is a Swift 6.2 SwiftPM accessory app for macOS 15 and later. It has a C transport shim, a core library, and an executable. It is an LSUIElement app: no Dock icon and no menu bar item. Feedback is a non-activating SwiftUI HUD.
A CoreGraphics event tap intercepts Volume Up, Volume Down, and Mute. That tap requires the Accessibility grant. Volume and mute travel as DDC/CI over I2C through the private IOAVService entry points, resolved at runtime with dlopen. The macOS default audio output is resolved to exactly one framebuffer. Eligibility is fail-closed: if the destination is missing, ambiguous, or not a controllable external monitor, the keys stay with macOS.
Those private entry points have no Apple contract. The default-output binding is recorded in ADR 0001. The app-owned HUD, rather than the private Tahoe OSD, is recorded in ADR 0002.
make ios-ipaThe command builds .build/ios/MonitorRemote.ipa. With it, the iPhone raises, lowers, and mutes the volume while it is on the same Wi-Fi as the Mac and the Monitor Remote app is open. The app lets go of the Mac when it moves to the background and reconnects when it is opened again.
The companion is installed with SideStore and a free Apple Account; no Apple Developer Program membership is needed.
After make ios-ipa, move .build/ios/MonitorRemote.ipa to the iPhone (AirDrop or Files), then open it in SideStore, which signs it with your Apple Account.
Idle measurements of the installed Release app on Mac14,9, macOS 27.0 build 26A428, arm64. Physical footprint is the quantity Activity Monitor displays, not RSS.
The numbers come from make profile. They are not a historical HUD record.
make buildThe Makefile builds, signs, installs, and launches Monitor Volume in ~/Applications. With an Apple Development identity installed, the Accessibility and Local Network grants survive rebuilds; with the ad hoc fallback, macOS may ask for both again after each rebuild.
make profileThe command measures the installed Release app at idle: CPU percent after discarding the unpaired first sample, physical footprint, and installed bundle size. It refuses to measure when the installed app is not the live process. It writes docs/performance/idle-baseline.json and regenerates the chart above. A missing baseline fails the renderer; it never substitutes a value.