I don't entirely understand your issue. You want to play multiple music files (ogg/mp3) simultaniously? Why would you want to do that?

I simply want to support OGG (and maybe also MP3) in my code. Being able to play a single music file and multiple WAV soundeffects seem perfectly sufficient for game development. Also, I dont think it's worthwile to support a lot of music formats. Most games dont have an awefull lot of music so it's not a problem to convert all content to the right format.