Same problem here. The mp3 at this URL will not play in Safari 5.0.1:
http://s3.amazonaws.com/rebakery/mediafiles/202/tevan_-essence.mp3
No broken QT image, just doesn't load or play. If you download it and open it in Safari, it'll play--just like George Mauro's problem. (Although, for the record, his mp3 plays fine in Safari from the link for me!)
I tried the MP3 Validator per Klaus1 (passed). Per Carolyn Samit's recommendations, I'm at 10.6.4 and I repaired disk permissions, and the mp3 worked—once!—and now no longer works again!
Other mp3s work fine from Amazon's server. (See
http://s3.amazonaws.com/rebakery/media_files/160/susan.mp3) The Content-Type fields are identical (application/octet-stream), so I'm not sure what the deal is. Looking at the mp3s' metadata, the only differences are that the mp3s that won't play are encoded as VBR, and with LAME.
This is a nasty bug!