Background Music

OS X audio utility

Automatically pauses your music player when other audio starts playing and unpauses it afterwards
Per-application volume, boost quiet apps
Record system audio
No restart required to install
Runs entirely in userspace

Version 0.1.0, first release. Probably very buggy. Pretty much only tested on one system (a MacBook running OS X 10.11 using the built-in audio device).

Requires OS X 10.10+. Might work on 10.9, but I haven't tried it.

Auto-pause music

Background Music can pause your music player app when other audio starts playing and unpause it afterwards. The idea is that when I'm listening to music and pause it to watch a video or something I always forget to unpause it afterwards. So this keeps me from wearing headphones for hours listening to nothing.

So far iTunes, Spotify, VLC and VOX are supported, but adding support for a music player should only take a few minutes (see BGMMusicPlayer.h). If you don't know how to program, or just don't feel like it, create an issue and I'll try to add it for you.

App volumes

Background Music has a volume slider for each app running on the system. I mostly use this to boost quiet apps above their normal maximum volume.

Recording system audio

With Background Music running, open QuickTime Player and go File > New Audio Recording... (or movie/screen). Then click the arrow next to the record button that looks like ⌄ and select Background Music Device as the input device.

You should be able to record system audio and a microphone together by creating an aggregate device that combines your input device (usually Built-in Input) with Background Music Device. You can create the aggregate device using the Audio MIDI Setup utility from /Applications/Utilities.

Install

No binaries yet, but building should take less than a minute. To build and install everything, clone/download the project and run the build_and_install.sh script. Unfortunately, it won't build if you don't have Xcode installed because xcodebuild doesn't work on its own anymore.

The script restarts the system audio process (coreaudiod) at the end of the installation, so you might want to pause any apps playing audio.

Uninstall

Delete Background Music.app from /Applications.
Delete Background Music Device.driver from /Library/Audio/Plug-Ins/HAL.

Optional

Delete BGMXPCHelper.xpc from /usr/local/libexec or possibly /Library/Application Support/Background Music.

Unregister BGMXPCHelper, delete its user and group, and delete its launchd.plist:

sudo launchctl bootout system /Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist
sudo rm /Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist
sudo dscl . -delete /Users/_BGMXPCHelper
sudo dscl . -delete /Groups/_BGMXPCHelper

Pause apps that are playing audio, if you can.

Restart coreaudiod:

sudo launchctl kill SIGTERM system/com.apple.audio.coreaudiod

Go to the Sound section in System Preferences and change your default output device at least once. (If you only have one device now, either use Audio MIDI Setup.app to create a temporary aggregate device, restart any audio apps that have stopped working or just restart your system.)

Troubleshooting

If Background Music crashes and system audio stops working, open the Sound panel in System Preferences and change your system's default output device to something other than Background Music Device. If it already is, it might help to change the default device and then change it back again. Failing that, you might have to uninstall.

Known issues

VLC automatically pauses iTunes/Spotify when it starts playing something, but that stops Background Music from unpausing your music afterwards. To workaround it, open VLC's preferences, click Show All, go Interface > Main interfaces > macosx and change Control external music players to either Do nothing or Pause and resume iTunes/Spotify.

Similarly, Skype pauses iTunes during calls. If you want to disable that, uncheck Pause iTunes during calls on the General tab of Skype's preferences.
Plugging in or unplugging headphones when Background Music isn't running can silence system audio. To fix it, go to the Sound section in System Preferences, click the Output tab and change your default output device to something other than Background Music Device.

This happens when OS X remembers that Background Music Device was your default audio device the last time you last used (or didn't use) headphones.
A recent Chrome bug can stop Chrome from switching to Background Music Device after you open Background Music. Chrome's audio will still play, but Background Music won't be aware of it.
Plenty more. Some are in listed in TODO.md.

Related projects

Core Audio User-Space Driver Examples The sample code from Apple that BGMDriver's based on.
Soundflower - "MacOS system extension that allows applications to pass audio to other applications."
WavTap - "globally capture whatever your mac is playing—-as simply as a screenshot"
llaudio - "An old piece of work to reverse engineer the Mac OSX user/kernel audio interface. Shows how to read audio straight out of the kernel as you would on Darwin (where most the OSX goodness is missing)"
mute.fm, GitHub (Windows) - Auto-pause music
Sound Siphon (non-free) - System/app audio recording, per-app volumes, system audio equaliser
Jack OS X - "A Jack audio connection kit implementation for Mac OS X"
PulseAudio OS X - "PulseAudio for Mac OS X"
eqMac - "System-wide Audio Equalizer for Mac OSX"
Zirkonium - "An infrastructure and application for multi-channel sound spatialization on MacOS X."

License

GPLv2 or later