For the last three or four weeks, sound just goes missing in heavy fighting situations in Cyrodiil. First thing to go is all battle sounds, followed by music and NPC soundbites until the fight goes on in utter silence. People reguarily ask in zonechat whether anybody else suffers from repeated sound-blackouts.
It always happens in heavy fighting situations with several eyes of the storm active. Sound slowly returns when retreating from the fight or when some of the heavy eye-of-the-storm-spammers die.