JDS Labs Headphone Amp with Arduino Control
It has digital volume control (DS1882) and the firmware is Arduino based. (Analog volume control with a digitally controlled potentiometer)
And it is open source, Hurray! [link]
The DS1882 is a dual, nonvolatile (NV) digital potentiometer designed to operate in audio systems that accomodate up to ±7V signal levels. The potentiometer settings can be stored in EEPROM so that they are retained when the power is cycled.
Arduino is also used to measure the battery level and indicate a low battery condition.
There is more: power supplies are based on the TPS7A4700 and TPS713301. The latest ultra-low noise LDOs from TI
Check it out here: [link]