Audio files in Firefox

Students build webpages in which are links to .m4a files, exported from Garage Band. With most students, audio files play smoothly when the link is clicked. With a small number of students, Firefox presents a dialog offering to play the file in iTunes or Save to Disk. Apparently Firefox isn't able to play the file.

So far I have ascertained that the problem is almost certainly something to do with the managed client and possibly something in the user's library. When the student logs in locally and mounts his/her home directory manually, Firefox plays the audio file no problem. Similarly, another user linking to the affected user's website (in Sites folder) has no trouble getting the audio file to play. The problem is independent of machine - have tested that also. The problem follows the affected user and unaffected users can succeed on an eMac that just failed for an affected user.

The question is what would cause Firefox to be unable to play the audio file? I have tried trashing Firefox prefs and any other possibly related files in the user's Library, but so far have been unable to change the Firefox behavior for the affected users. 

Any ideas? (And when will Safari be able to play mpeg4 files?)

TIA

Peter

Peter Monether

Technology Coordinator

South Meadow School

Peterborough, NH 03458

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Associate Faculty, Education Dept.

Antioch University New England

Keene, NH 03431