Same here, although for me, it's only when sims are playing instruments. Audio cuts in and out. I can hear what they're playing for a few seconds here and there, but for the most part, I've got sims rocking out to...silence.
Audio in other parts of the game (sims talking, dogs barking, etc.) can still be heard, just not the music coming from the instruments sims play. It's a shame. One of my sims is working on the musical genius aspiration. She's a level 10 guitarist, so I'm sure whatever she's playing would probably sound pretty good, but considering how long it takes sims to write songs, all of those hours of silent songwriting have made for a pretty mind numbing gameplay experience.
The same problem applies not just to the guitar, but also to the piano and violin. If I'd realized that instruments were silent, I wouldn't have made her a musician. Playing instruments is pretty pointless if there's no music coming from them.
EDIT: Well, she got nominated for an award when she licensed the last song she wrote, so it must have sounded pretty good, not that I'd know. :/