Check your audio settings and make sure it's set to a single speaker or stereo. If it's set to 5.1 surround or something and you don't actually have that type of setup, some sounds may be missing.
Thanks, Hawkeye22! That did the trick! My setting was on 7.1. How it got that way I've no idea, but I reset it to 2.1, my normal setting, and it works fine now. Thanks a bunch!