I just installed a xmms plugin to play m4a. I was looking for the name of the plugin or codec needed and this post gave me the answer.
Firts enable livna repos
Go to http://rpm.livna.org/rlowiki/
and click on the right version or just get the file with a command like:
and install the aplication... I did a double click on the file and typed root password when I was asked. But I think that you may use the following command as su.
rpm -i livna-release-9.rpm
Then I installed the package that I wanted from livna, I did the whole thing in one line (again as su)
yum install xmms xmms-mp3 xmms-aac
Actually it does not installs xmms-acc but instead installs xmms-faad2.
I really didn't care much, as finishing the install I opened one m4a file and it worked.
Hope it works for you