Try an opto-coupler to interface the amp output to the rest of the circuit.
Use a variable resistor on the input of the the opto-coupler.
Use a pull up resistor on the output and a second transistor to invert the pulses. Your Arduino can do the rest.
When no second transistor is used. programming the Arduino to work in reverse .... eg 0 = to 255, and 255 = 0. Use one of the analog inputs and write to the Digital outputs.