Well, shit. I was asking all these questions assuming that the game wasn't overriding the wav file properly, but the problem is the game thinks there's something wrong with the wav file. I opened it in audacity and simply re-exported it to see if it would fill in whatever header or whatever the game thought was missing, and it seems to be working properly. I guess Quake just uses an old and/or hacky wav format or something.
Sorry, man, my first question should've been to clarify what the actual problem was. Anyway, here's the re-exported version.