Taken from the Japanese Wii Music site:

Ad Lib Performance
– Choose among 50 tunes, 60 instruments, and a selection of stages
– As you play alone, you can record 6 different instruments separately and mix them

Share clips with friends
– Record your performances
– Design an album cover for each record
– edit performances that you’ve received
– send to friends

Lesson mode: Basic Play
– Learn the fundamentals of the game
– practice with all instruments to become familiar with them

Re-arranging Lessons
– Learn to choose the appropriate instrument depending of the the type of song, as well as the basics of conducting a musical performance

Drum Lessons
– Use both hands (Wiimote + Nunchuk) and your feet (Balance Board) to learn how to play a drum set

– Minigame 1 : Conduct a Classical Orchestra
– Minigame 2 : Connect the Handbells
– Minigame 3 : Test your Musical Sensitivity