I am mixing two stereo audio signals together and would like to control the output volume using the (0-3.3v) DAC on my microcontroller. I'm wondering if there is a simple modification I can do to this circuit so that it accepts a 0-3.3v reference for volume or would I need an additional opamp stage? If additional stage is needed, what would you recommend as it's configuration?

[![Mixer Circuit][1]][1]


  [1]: https://i.sstatic.net/eKVM8.png