I think this is all of them:
Keyboard
Trumpet
Timpani
Guitar
Mic
Headphones
Piano
Saxophone
Bass Drum
Harp
Boom Box
Turn Table
Accordion
Drum Set
Violin
Singers
Trombone
Karaoke
Mega Drum
There is one more potential instrument: the wild guitar. I've seen photos people have posted of it, when it appears in their MusicStar shop, but I haven't seen it myself.