An Example:
If you want to replace the normal music in the game with your song than save the file backgroundSinglesMix.ogg in the folder Sound by renaming it to backgroundSinglesMix.ogg.bak. Then you rename your converted titel to backgroundSinglesMix.ogg and copy it in the directory. Done. When you now start the game, your music will be played.

Freeware to convert different formats:
dBpowerAMP Music Converter (dMC)
And the right codec:
Codec Central - Ogg Vorbis

Freeware to merge musictitles:
musiCutter