The AVR Butterfly port of One Bit Groovebox is a portable synthesizer. The hardware is a AVR Butterfly board with an additional potentiometer. The software is a slightly refactored code from the original One Bit Groovebox with additions incorporated to support on-board peripherals.
OBG-BF.zip is the archive with the source code. Read the ReadMe.txt file (also in the archive). Make the hardware addition as described there. Load the OBF-BF.aps file into AVR Studio, build the project and program the board.
You may also contribute to the original code, found at https://launchpad.net/bitbox/ . You may even try to integrate the source code from the archive above into the trunk.