This happens to me. I have to keep the Music volume setting for the game set to Mute, and I don't have the issue with volume suddenly exploding that way. It's not really a fix, since that means I can't hear music in the game. I also suggest making sure your game is fully patched (I use the patcher from ModTheSims and can link it to you in a PM if you'd like.) I used to have the game music come on even if it was muted, and instrument/TV/video games/etc would all be randomly loud, but haven't had those problems in a while. It could be a patch that fixed it, or one of the mods I use. I only have AwesomeMod, nraas' Overwatch and Error Trap, and nointro. So... -shrug-