You can upload your own Midi's can't you? watch a vid where there was a group doing a trio, it was funny and amazing.
Think he was talking about the ingame music rather than the instruments.
Though yeah, there are sites for downloading the abc files that the instruments need so you can play loads of stuff in game. Saw a video the other day of Always Look On The Bright Side Of Life with 4 people stood on crosses in a desert planet