So you're saying that this is related to a problem with the game software itself?
Well, as a point of comparison: I haven't upgraded yet (still just Uni and NL,) and I have the same problem all the time. I tend to let them turn on the TV and the stereo and keep them on all the time (to prevent napping outside of bed,) and I've noticed occasional silent piano-playing, or the stereo going silent when they take a shower.
The game just doesn't play multiple sounds well.