Weirdly, though QuickTime 2.1 gives me a bus error on boot ever since installing the accelerator (it worked previously), I tried QuickTime 3 on a whim and it works absolutely fine, with Myst successfully playing videos and Doom successfully playing music. I really don't understand what's going on with this machine.
I've tried disabling Mode32 to have it only use the subset of RAM that it was able to access before the accelerator went in, but that didn't have any effect on things either. I haven't yet tried putting the original RAM sticks back in.