Avr_theremin is a project mainly written in C and ASSEMBLY, it's free.
A theremin built from a microcontroller and IR proximity sensors. Based on http://www.nerdkits.com/videos/theremin_with_ir_distance_sensor/
Based on http://www.nerdkits.com/videos/theremin_with_ir_distance_sensor/ with a few modifications.
To build the firmware run make in the software subdirectory. Flash it to the chip with make prog.
The hardware subdirectory contains Eagle files for our own PCB design.
Have fun!