Start up a multiplayer session (single pack) witha friend. Choose your song, instrument, and begin playing the song. Have your friend close his DS, placing it in sleep mode (assuming you are the one with the cartridge). It'll say there's been a connection error, but even if you turn off your DS, your friend will still have the game on theirs and as long as they don't turn off the DS via the power button (sleep mode works great) they'll be able to play the song you selected over and over.