For a while I have been running XBMC 13.0 on CentOS-6. I turns out that there is a known issue in that version which prevents it from playing .m4a files.
The solution is to upgrade to 13.2 which for me means rebuilding a new RPM.
For you, that means downloading the new version from http://www.chrysocome.net/downloads/xbmc-13.2-1.el6.x86_64.rpm
And don't forget to get the deps from http://rpmfusion.org/ and EPEL.