What's Vorbis ogg :
http://en.wikipedia.org/wiki/Vorbis
To sum up, it's just another file format like mp3 but better and open-source.

And if you are thinking of converting MP3 to OGG it's a bad idea since you'll have quality loss in the convertion. You should always convert from WAV if possible.

Audacity can do a really good-quality conversion, it's really if you EDITED the mp3 with something that the quality will drop.

Try the FREE WMA/MP3/OGG converter! (something like that.)

It converts more than the ogg files for all your filing needs.

So only .wav files can be converted into .ogg files?

Let be clear.. any audio file can be coverted to any other... given the right software.

Audacity is good because it has support for ogg and pretty much every other audio format.. ever. and it's free.

xanax's thing about MP3's is probably not going to affect you...
The Quality issue with MP3 only really applies below about 160kbps... anything above that and you really won't notice the difference.
But if you originally got (or made) the file as WAV then convert from that... if you've only got MP3 it should still be fine.

Another question, is it possible to change the music/loop sound of the level?

Uuuh, yeah!
Click the "Set music"/"Set Loop sound" button in WooGLE, then browse and find the right file...

Or do you mean, while the level is in progress? If you mean that, then no. The only case this happened in the original game was in GeneticSortingMachine, and that music change is hardcoded into the .exe.

If you want to do this, you could use custom utility balls to play sounds. You could trigger and stop them with some kind of mechanism like in "gotcha!"
The problem is (as far as i know) that balls doesn´t loop sounds, so you´ll have to loop it yourself and make it long enough to avoid soundless levels.