Never mind, I found the problem: a loose headphone plug. Although why
this would make mp3 files sound like they been put through some kind of
Karaoke converter with music sounding nearly normal but all the vocals
coming out almost indistinguishable is very strange. And it wasn't just
faint, but had extreme reverb.

Ah! I can tell you exactly why.

Thanks to the partially withdrawn plug, the sound was being sent through the
two earpieces in series, but you were only hearing one channel. The vocals
must have been on the other channel.

OR... if for some reason you lost the ground connection but kept the right
and left channel connection, you would actually get a karaoke conversion.
The signal would go through the 2 earphones in series, but you would only be
hearing the difference between L and R. That is exactly how karaoke
conversion works -- they remove what is identical in the two channels, and
that zaps the lead vocal, which is normally right in the center.