The Arduino Audio DSP Shield has ring modulation, echo, reverb, an oscope, oscillator, arbitrary waveform generator, etc. It is a software and hardware system, a sound man, in a can!
Step 1: Get an Arduino Audio Shield
The shields are available at kickstarter http://tinyurl.com/AudioShield
at a low introductory price. They come fully assembled, with software. Schematics are available to backers.
Step 2: Get a Case
The cases are available in clear plexiglass, and come with hardware and instructions for easy assembly.
Step 3: Get the Gold!
The gold cases look really spiffy and are available for showing off your new arduino dsp shield. New software is being developed for the shield even as you read this.