Before I emailed Pablo Gian Villamil and he sent me on a link to his class notes (yay!) for Generating polyphonic sound with Arduino I tried to work out his circuitry from his various blog posts and component listings…

• CMOS Hex Schmitt Trigger Inverter Integrated Circuit (74C14) or CMOS Quad NAND Gate Schmitt Trigger Integrated Circuit (CD4093) (on the board but not using it 4093 – 4093B Quad 2-input Schmitt trigger NAND gate – http://www.doctronics.co.uk/4093.htm)
• CMOS Binary Counter/Divider (CD4040)
• AD5206 – 6-Channel Digital Potentiometer – Controlling a Digital Potentiometer Using SPI
• various capacitors – 4–10uf. .1uf, 10uf
• 1MOhm pot
• LDRs

Google searches on theme uncovered several things of interest…

heuristicmusic’s posterous – 74C14 Square Wave Oscillator – “This weekend, I started working with Nicolas Collins‘ excellent book Handmade Electronic Music (Routledge ISBN: 978-0-415-99873-4). One of the more intriguing projects in the book is an amazingly simple, inexpensive square wave oscillator based on a $0.75 74C14 CMOS Hex Inverter chip.”

Nicolas Collins – Handmade Electronic Music – The Art of Hardware Hacking
http://www.amazon.com/Handmade-Electronic-Music-Hardware-Hacking/dp/0415998735/ref=sr_1_1?ie=UTF8&s=books&qid=1257730334&sr=8-1

PROJECT CIRCUIT DESIGN – Square Wave Generator Using CMOS 74C14 Single Integrated Circuit Chip (popup window hell!)