Using a rotary encoder as a volume control
This describes how to use a potentiometer with a MCP3008 chip.
Hardware
Potentiometer from Ebay: http://www.ebay.com/itm/10PCS-6mm-3pin-Knurled-Shaft-Single-Linear-B-Type-B10K-ohm-Rotary-Potentiometer-/222445944546
MCP3008 from Adafruit: https://www.adafruit.com/products/856