Arduino potentiometer (external power) with amplifier
It's impossible. The pot is most probably acting as a passive variable attenuator. As such the top end will be the incoming audio signal, the lower end will be ground, and the wiper will be the attenuated incoming signal. There's not a lot you can do with that.  

You would need to replace the pot with an equivalent triple pot (if such a thing exists) and have one third of it connected to the Arduino.  The pot you have at the moment is a dual pot, and a log taper at that.

Alternatively adding some kind of quadrature (motor) encoder to the shaft of the pot may be a possibility.