Fix audio error related to fading ambience
Fix audio error related to fading ambience

Prevent audio manager from trying to fade out ambience upon return to
the map screen if the sound has already finished. This was somehow
delaying the stadium pop-in, likely due to interactions within the
createJS library.

tiltfactor/smorball,tiltfactor/smorball,tiltfactor/smorball,tiltfactor/smorball