I assume you are using Windows ... m4a is an Apple file type, and I am probably guilty of sharing some without realizing they can be a problem I'm sure there is a way to play them on Windows, but you may have to investigate a bit.

I found this page on Google, which may be a good start.

http://www.osxhax.com/archives/000046.html

Let us know if this helps.

__________________
To get Phex Click on the Fox