[NatureTM] sent in a writeup for the MIDI synthesizer he recently built using a TI LaunchPad. The construction is pretty simple overall, consisting of little more than the MSP430, a handful of resistors, and an optoiosolater. Of course, a MIDI controller is required, but he already had that on hand.
Once the MIDI data is read from the optoisolator, his code takes care of the rest, adjusting the square wave sound generator to get the proper note. He does mention that since the synthesizer is monophonic, special care is taken to ensure that simultaneous notes are processed properly. You can route all of the sound into a single speaker, but he used the optoisolator to send the sound data to multiple LaunchPads instead, resulting in a fun little MIDI quintet.
He provides code and plenty of video on his site, but keep reading for a sneak peek of his synth in action.