are you using iTunes as your main mp3 player or quicktime?? (& why would you use quicktime for this??)
first, time to rebuild the desktop (since itunes HAS icons for mp3s...)
you know, hold command-option thru the startup, then click "yes" when it asks if u want to rebuild...
if you're interested, there's a little app called "itunes associater" (get it
here that will convert ALL your mp3s (in groups, or singly) it iTunes files.
there are also apps at versiontracker.com that can convert files to other formats (ie quicktime)
hope this helps!