Volume Control for Mixer Circuit using DAC Refence
While you could, with some effort, build a voltage-controlled amplifier, let me suggest a somewhat different way to accomplish volume control + mixing. IC manufacturers provide a rich set of devices for this function as some form of volume control is required in just about every consumer audio product out there.

The Texas Instruments LM1971 is a digitally-controlled audio attenuator. So instead of using your microcontroller's DAC, you would hookup digital signals to the IC's LOAD, DATA and CLOCK pins.

There is no need for your OPA1662 input stage. Simply connect the left and right audio signals to the point labelled Audio Input with 1K to 5K resistors. I'm assuming your audio signals can drive that impedance. You'll still need the 1uF cap.