In order tp playback M4A files with XMMS you will need a plugin.
I am just uploading a rebuilt "faad2" package (for Slackware 13.37) which includes a xmms plugin for the playback of m4a audio.
Get the package here: http://slackware.com/~alien/slackbuilds/faad2/
. You will need a second package as well: id3lib which you can get here: http://slackware.com/~alien/slackbuilds/id3lib/
If you are not running Slackware 13.37 you can just grab the SlackBuild and other source stuff available in the same location and compile a faad2 package with xmms plugin yourself.
I verified on my laptop that installing my faad2 and id3lib packages will enable xmms to play m4a audio straight away.