Import raw MIDI data, and hit the green flag to play it. Not the greatest MIDI player in the world, but it's OK for scratch I guess. Up to five tracks are allowed (any more and the project will lag severely).
Note: If it falls behind at all, it rushes to catch up. I highly recommend playing it on Turbowarp at 240 FPS. To change voices, all you have to do is click see inside and change the "set instrument" block in the desired track. (track 4 is for drums. Depending on where you got your data, the drum sounds may not match up.)