Adding music to existing 5.1 soundtrack

I'm not sure if this question is more suited to FCP forum, but I have a 5.1 segment (L+R, C, RS+LS, LFE) in FCP and I would like add a song from iTunes as overlay to this. All channels currently have sound f/x and center has dialogue but no real music can be heard.

Can someone provide me an tutorials on how to do this?

Thanks.