I'm not sure if this is a glitch or not, but there is very little sound or music in my game. The voiceovers are fine, and there are no problems with cutscenes, but I find it disorienting to watch Feeble do something like kick a vending machine and not hear any noise. Is it supposed to be like this? If not, what should I do?

I don't have the GOG version (when I bought it, that wasn't an option), but I get sound effects when I play the game with ScummVM. I think the only music in the game is the one in some of the cutscene movies.
I don't have any realistic ideas about what could be causing your problem, though. (ScummVM allows you to change the sound effects volume, and if it's set to 0 there will be no sound effects. But why would that have happened here?)

Update: I tried running the game in Administrator mode. Now I'm getting a lot more sound, and ambient background noise... but apparently the lack of music isn't a glitch. Oh well...