
MSSIAH
"MIDI cartridge for the Commodore 64. Cheapest and easiest way to make your C64 MIDI compatible."
PROPHET64 - Discontinued (replaced by MSSIAH)
SID2SID - "Do-it-yourself circuit board. Helps installment of a second SID audio circuit in your Commodore 64. Brings stereo or up to six channels of polyphony with the MSSIAH."
Prophet 64: Sequencer Demo Song
YouTube via skrenos
"This is the Prophet 64 cartridge running on a Commodore 64c with dual SIDs. SID1 is a 8580 and SID2 is a 6581R2. I have lowered the frame rate to 1 FPS as the audio is the most important part.
This is just to show you what kind of sounds you can get out of the Commodore 64 with the P64 program."
via Mr. Array