Well, the idea isn't that people upload the songs to
your server; they upload somewhere like fileden or their own server, and then they link to the MP3 like a url tag.
I just took a look at your bbcode help page (
http://www.wachos.cl/comunidad/misc.php?do=bbcode#mp3) and your example works just fine (except you should rename the song title wrapped in the tags; I used Even Flow as the example song on my site). What are you having trouble with?