Digitally controlled audio fader circuit?
Or you can use digital pot chips like AD5242 series dual I2C digital pots, coupled with some op amps.

Here is a reference design for you:







simulate this circuit &ndash; Schematic created using CircuitLab

The two pots there is the digital pot chip AD5242BRUZ10 commanded by the Arduino. You can program the Arduino to handle MIDI interface and plug this into your professional audio system rack.

OPA4134 is a proper audio grade op amp but it is expensive. You can experiment with orders of magnitude cheaper but pin compatible LM324, or get rid of the two voltage followers and use the cheaper OPA134 audio op amp that is pin compatible with 741. 

TL431 is a 2.5V reference chip used as a virtual ground here.