I'm not sure about synaptic, when I did this I used "sudo apt-get install
ubuntu-restricted-extras"... check this page for more info:
https://help.ubuntu.com/community/RestrictedFormats
If I remember right, this will set you up for playing dvd's and have flash
and Java working in-browser as well as fixing the music issue. I do know it
is possible to just select codecs for mp3 player, and the next site suggests
a way to get that:
http://www.psychocats.net/ubuntu/mp3
But that method didn't work for me.