Assuming you are running OS 9.0 + but not X, you will should be able to play all mp3s by doing two things:

1st: Ensure that you have Quicktime, (5.0) preferrably. If you do not you
can obtain the free version from --
"http://www.apple.com/quicktime/download/".

2nd: After making sure that you have quicktime, you must then make sure
you have the correct file mapping settings for the Mp3 file format.

A: Go to the Apple menu at the top. Go down to "Control Panels". Go
down to "File Exchange". Select the "PC Exchange" tab. Go down
and look for "Mp3" under the PC Extension heading. ( Should be in
order, if not click on the PC Extension heaing to sort).

B: Once found, ensure that "Quicktime Player" is selected for all
entries of Mp3. All should now work upon clicking on your Mp3.

C: If you don't find Mp3 under PC Extensions -- Click the "Add" button
-- selecting "Quicktime Player" as the Application -- "Mp3"as the
File Type.