You might like SuperJam. You can set a "band" playing in a number of different styles and then play notes or chords on top of that with a number of different instruments. MIDI compatible, very powerful, especially when paired with Bars and Pipes. A little tight with 1MB of RAM, but it should work (get yourself a trapdoor expansion to go up to 2MB).
DMCS is a more "traditional" kind of music program in which you put notes on a staff. Very easy to use. Deluxe Music 2.0, its successor, is my go-to whenever I need to sound out a couple of notes.
