Today I started troubleshooting by replacing the speakers with earphones. The sound was great! Yay, I'll buy new speakers. So I happily listen away to mp3s in Winamp 5.13 and start ripping CDs therein. Disaster! The sound is borked again! Same problem as before.
Straining my memory, I think the first breakdown may have been after I was ripping music in Winamp 5.13.
That's as far as I've gotten. What things should I now check out? Reinstall Winamp? Reinstall mp3 codecs? Buy new speakers? Buy a new sound card?