..or based on the current attention mode of the user.
watching a movie with my girlfriend, i don't want calls to break audio or IM bubbles to jump into Batman's face. that's something we are not yet addressing, and also the settings in the Me Menu don't quite cover that yet..
Busy aka Do Not Disturb perhaps.. yeah.
and in that mode, i wouldn't want any blips or bubbles.
the only case i can think of as being relevant is a warning about imminent hardware failure or such..
otherwise, read ahead the sum of active audio streams, calculate RMS and duck[1] it to the notification sounds. if notification sounds are designed well, then we DO want to hear them.
so now that they are actually helpful and don't suck anymore, please let me know somehow, if they are muted.
[1]
http://www.sweetwater.com/expert-center/glossary/t--Ducker