He is asking how to loop Ogg music files not how to convert Mp3's to Ogg.
All you have to do is multiply the place you want your song to loop by 44100 Hertz.
For example. I have a song thats 1 minute long(60 seconds). I want it to loop at 10 seconds. So all I do is multiply 10 by 44100 which should get you 441000.
Now you need something like Winamp to add the looppoint. Download Winamp, if you don't already have it, and open up your music file with it. Find your music file, right click it and click on where it says 'View file info' or something similar to that. Find where it says Comments, which should be under the 'Basic Info' tag and type in LOOPPOINT=(followed by your loop point number, which is 441000 in this case).
All you have to do after this is just add it into your wad and the music should loop.