Thanks, that looks very good, maybe i'll buy it
So, you said i can do it without the shield, but limited.. How?
There is a third-party Tone library that can play two simultaneous square-wave notes on different pins:http://code.google.com/p/rogue-code/wiki/ToneLibraryDocumentation
You could generate square waves in software to get multi-part harmony. We did that back in the Intel 8080 days (1975 or so) to do four-part harmony. I expect the ATmega would be at least as capable. Every clock tick you add a constant to an integer and if there is an overflow you invert an output pin. A constant of 0 gives you silence. A high value gives you a high pitch (1000 give you about clock/65.536) and a low value give you a low pitch (65 give you about clock / 1000). Minimum pitch is 1 (clock * 1 / 65536) and maximum is 65535 (clock * 65535/65536). You could use unsigned longs to get even more resolution.