User taylore posted on the MacFixIt forums (and gave permission for me to recreate here) a problem he noticed with Classic and sound output. Here's what he wrote:
"As users of Classic may know, you stop hearing sound output by Classic if you are playing sound from OS X simultaneously. This, for example, may explain why you're not hearing Eudora's New E-mail warning--particularly if you are playing MP3's through OS X.
Here's one I just discovered though:
If you have an MP3 selected in the frontmost window of the OS X Finder it will have the same effect!
The MP3 file doesn't have to be playing, the mere fact that it is selected will stop all sound output in Classic. Grrrr. Anyways, if you're having trouble getting sound to play from Classic (and it has been working previously), there's another thing to add to your checklist while troubleshooting."
The glitch has been reported to Apple...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20001213002715474