It seems that the latest couple versions of rhythmbox has had xine support removed the ebuilds. Unfortunately, this is required to play .m4a files (generated with iTunes). I would request that xine support be enabled until gst-plugins-faad can be used to play iTunes generated .m4a files.
The actual error rhythmbox throws up when trying to use gstreamer to play .m4a files is that there is no handler for audio/x-m4a mime types.
Steps to Reproduce:
1. emerge gstreamer-0.8.3
2. Try to import an iTunes generated .m4a file (you won't be able to)
3. If you already had one in your library, it won't play.
rhythmbox no longer supports .m4a files.
rhythmbox should be able to use the xine backend to play .m4a files.
Oops, step one to reproduce should be emerge rhythmbox-0.8.3, not gstreamer...
xine support has been removed because there is a lack of maintainership for it. Please wait while we try to get this sorted out on our side.
and this is described in the ChangeLog