I don't know but I can explain it to you, it's really simple.
Just, before you go into the process, download audacity and download brawlbox if you don't have it already.
Whatever song you desire, let's call it
http://www.youtube.com/watch?v=0SxrxQy3HO0 this for example.
What you would do is download the MP3 of this however you can. Once you have the MP3, Open Audacity, and you can adjust the volume/start and endpoints of the song/speaker shift/bass boost reduction, literally WHATEVER you want or desire. Audacity is an excellent music editing tool.
When you're done ****ing around with the song and have the right adjustments, EXPORT IT AS A WAV FILE, Brawlbox doesn't read anything besides .brstm and .wav files. .wav files being what I just mentioned, and brstm files being what you'll put on your SD card. I'm sure you know this.
Anyway, with wav file in hand, open brawlbox, and go to file, new, and go to brstm audio stream. Select the wav file, and then a window will open. What you want to do here is set up your looping points. the bar on the left being where your loop will start (don't confuse it with where the song will start, you may or may not have edited that with audacity, but it'll always start at the 0:00 point of your song, and will go up until the "loop end" point. from the loop end point, the song will then go to the loop start point.)
So to be a perfectionist like I kinda am sometimes, I like to have flawless looping, so i'll sit for as long as 10 minutes until I find a decent/good/perfect loop point. to create the loops, just press the button with * on it. and be sure to click the loop checkbox in the bottom left, otherwise your song will play from start to end with no loop.
with the final brstm you want, just save it and unleash your musical wrath. If you don't like it or would like to adjust something like the volume or speaker ratio or something, then you can go back to audacity and mess with the wav. to remake the brstm, all you need to do is copy and paste the numbers you see on LoopStartSample and NumSamples, which are the start/end loop numbers respectively.
this is probably an extremely ****ty guide but im kinda in a rush. I put down the majority of the basic stuff. with experience/just messing around with a song a few times/frustration you'll learn to be perfect. trust me loops are important, otherwise the song just plays extremely awkwardly, or transitions into a weird part of the song and doesn't sound right if the loop is bad.
edit: wow i linked that song and didn't even use it as an example. so yeah, i would probably turn up the volume a bit on that, maybe +3 or so, but be careful, if you turn the volume up too much it'll become really statical and you don't want that. or maybe you're into that type of stuff. idk. but yeah, turn the volume up. then open it in brawlbox, and as looping points, i'd take the end loop around 2:00 as an end point and go to :24 as a start. the song i linked is really simple, and if you listen to it once if you're good or twice as an average number, you'll be able to tell/see/hear good points to loop the song at.
and i mean, as to where you'd place the thing...tbh it's up to you. whatever you feel like would give you the best use out of that song. I'd put it on the menu, but that's just me. I don't have that song on Brawl and I'm probably not gonna put it on, but it's a good example.