Background Music
Automatically pause your music, set individual apps' volumes, etc.
...Background Music provides a volume slider for each application running your system. You can boost quiet applications above their maximum volume. You can record system audio with Background Music. With Background Music running, launch QuickTime Player and select File > New Audio Recording (or New Screen Recording, New Movie Recording). Then click the dropdown menu next to the record button and select Background Music as the input device. You can record system audio and a microphone together by creating an aggregate device that combines your input device (usually Built-in Input) with the Background Music device.