As far as I know, there is no tool for importing music in G/S/C.
However, there is a music hacking document here that you can get started with: http://hax.iimarck.us/topic/1263/
I actually modified my old pointer tutorial today (got the idea to do it after I had answered your question about repointing data). You should check it out and see if you figure out how to do stuff after reading it: http://www.pokecommunity.com/showthread.php?p=7263429