Audio ducking for media?

Is your feature request related to a problem? Please describe.

Inferred from @eeejay feedback: When e.g. listening to music, and suddenly a notification comes, we'd want the music to get lower so we can hear the notification, but not pause the music, since the notification could be a minor thing.

Describe the solution you'd like

SD could use the mixer to duck the audio of other applications.

I'm wondering if SD should just silent everything else, or we'd want some filtering?