I decided to build a synth from scratch. Fortunately other people did a lot of the work for me. I found this guy on the arduino forums. He build an arduino based synth capable of four simultanious notes, controlled by MIDI. Perfect. (The current generation of his synth is much more awesome - check it out!)
For now, the synth recieves note on and note off events, and plays up to 8 notes polyphony. I will have to up it at some point - but that is a headache for another day.
There are a few options here
I bought a WaveBlaster daughterboard (NEC Xr385 - a clone of the Yamaha DB60XG), which i made a support circuit for. I found schematics for the circuit at the Electro-music forums. I suspect that the sounds are kinda crummy, after all it is a wave table add-on to a mid-nineties sound board. I hope to find a lot of retro sounds on the board - or i might just build it into a stand-alone MIDI synth module.
I have found a project hosted at GoogleCode: 4bitsynth. It appears to be just what i am looking for. It is a piece of code for an Atmega48 microprocessor, which recieved MIDI data and outputs either square, triangle or noise waves. In its default configuration it can sustain 4 simultanious voices. Maybe this can be expanded? (For it to be useful in this project, i need 6 simultanious voices).
I really havent looked much into this, but it could be great fun to maybe get hold of a couple of SID chips or other sound chips from the same generation. I remember something about one of the sound chips from the first generation TV-consoles having a larger polyphony, maybe this was worth looking into - maybe as a later upgrade project.
I plan to mount the support circuit into the accordion, allowing the daugtherboard to be dismountable. This only requires the 4bitsynth board to have a compatible connector and the WaveBlaster connector is well documented (for example at Wikipedia).