There is a know bug in Firefox on OSX 10.6 (and perhaps later), where embedded mp3 files do not display correctly.

The fix is easy (or you can just use a different browser!)

Go to Applications > Firefox
Right click (or control click) on Firefox, and choose "Info"
In the Info box that pops up, select "Run in 32 bit mode".
Close and restart Firefox.

Embedded mp3s should now show up properly on this site and on every other site.