First you have to convert it into an .ogg file. You can do that by opening the song in audacity and then export it as an .ogg file. In this folder Vista: C:\Users\"your username"\AppData\Local\Ironclad Games\Sins of a Solar Empire\ or XP: C:\Documents and Settings\'user name'\AppData\Local\Ironclad Games\Sins of a Solar Empire\
There will be some mod folders open the one that fits the version of your game and make a new folder with the name of your mod, could be star wars music. In that one make a folder named Sound. You then put your .ogg music file here. Now in order to make it replace the old music you have to name it exactly as the one you want to relpace.
You can find the original music in your install folder and then sound (if it is the expansion you want to edit, then first open he entrenchment folder then sound (note, don't make an entrenchment folder in your mod folder)). Then find the name of the song you want to replace (if there is more then just copy your star wars file and give them each their name). You can open and listen to the original music in audacity (though might be a little troublesome) or use vlc player. Now when you start your game go to options and your mod will be listed as you named it, which in my case will be star wars music, then enable it and apply.
