After some battles (seems to be always coop, can't confirm), the entire sound of the game stops. Muting/unmuting won't work, and its ALL the sounds, in battle or in city. It come back if I keep battling (not in city), sometimes all and others just partially (voices, but not BG sound). Refreshing the page always works to restore sound.
I'm using Mac OS X 10.6.8 with Safari 5.1.9