Well, you can use a program called vdownloader to download an mp3 from a music video on youtube, then use a program called winFF to convert it to a .wav file then go into brawlbox and select file-new-brstm audio stream, and select the wav then wait for it to convert it to a brstm and save.
Go here for what to name it on your sd card:
http://hardcorebrawlhacks.wordpress.com/2010/02/22/brawl-musis-hack-coding-reference/Oh, and put the file in /./private/wii/app/RSBE/Pf/sound/strm