For some reason, the music for all of my old games that run on DOS (like Tyrian, Star Control II, etc.) is all glitchy and plays really slowly, with distorted pitch and speed.
Does anyone have any idea what could be causing this?
Is running under emulation (i.e. DOSbox) not an option? Or are you already running these under emulation? In the specific case of SCII, I'd suggest getting the modern Windows remake ('The UrQuan Masters'), it's awesome (it's just like the DOS game but with the superior 3DO sound).
If your UQM has broken sound, you've got serious problems. It also doesn't fall under 'old games that run on DOS' at all: any problems with UQM sound are going to be wholly unrelated to legacy concerns.