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?