amps and audio circuits
http://www.ti.com/lit/ds/symlink/lm4861.pdf
http://forum.arduino.cc/index.php?topic=164436.0
http://sound.westhost.com/dwopa.htm
https://github.com/TMRh20/TMRpcm/wiki

http://avrpcm.blogspot.co.nz/2010/11/playing-8-bit-pcm-using-any-avr.html
http://extremeelectronics.co.in/avr-tutorials/sound-generation-by-avr-micro-tutorial-i/

http://www.uchobby.com/index.php/2007/11/11/arduino-sound-part-1/
http://www.righto.com/2009/07/secrets-of-arduino-pwm.html

http://highlowtech.org/?p=1963
http://www.arduino.cc/en/Reference/UsingAVR