On the Japanese Wii Music website, Nintendo has begun a series of videos called "Today's Musical Instrument," which depicts an instrument both in its for-real form and as it will appear in Wii Music. The first video features the marimba: after some introductory text, we're treated to footage of a model pretending to use the Wiimote and Nunchuk as mallets playing an invisible instrument. This is followed by Wii Music footage of the same instrument. It's a cute video and a happy little tune.
Like a lot of gestural Wii games, this is going to involve some playing-along on the player's part. You could just shake the controllers wildly and achieve pretty much the same results as making instrument-appropriate motions, but that sort of defeats the purpose of having different instruments in there.