I think the sound cuts out in Cyrodiil as a way to help avoid or lessen lag. Sound would cut out occasionally during a sizeable battles, hmmm .... over the past several patches. It did, however, seem more noticeable yesterday when I was in Cyrodiil (during a rather epic siege battle). While loosing sound sucks, at least I can swap skill bars timely, and at least when I press a skill it goes off.