The Echo firmware makes use of the Mozzi sound synthesis library for Arduino, and as such requires the library to be installed to compile correctly (get it at the link above).

This firmware is currently proof-of-concept only, and is provided here in case you want to experiment with it yourself.
Download the current version: Groovesizer_Echo_005

To run the firmware successfully, make sure you have the jumpers configured as follows:

Here’s a video introduction to the Echo firmware.