The new song pack will see the first use of the DS's GBA slot for something other than GBA games.
Nintendo of Japan has updated their Band Bros. DS page announcing that an expansion for the game is in the works. The "Band Brothers Zouka Kyoku Cartridge" will feature 20 more songs for the DS music game, and Japanese gamers can vote for which songs they want to see included.
The way these songs will be made available to DS owners is what makes this expansion interesting. The pack of songs will come on a GBA cartridge. Both the DS card and the GBA cartridge need to be inserted in the system at the same time for the new songs to be available in the game. This marks the first time that the GBA slot on the DS is being used for expansion purposes, and not just for Game Boy Advance games.
According to the game site, more information on the expansion will be made available on August 22, which will likely include a list of songs voted on by visitors to the Band Bros page. While the US version of the game, Jam with the Band, has been announced, there still aren't any details on when we can expect to see it in the States. Hopefully, Nintendo will release more info on it by the end of the year.