PJRC forums member rwalters wanted to build an analog mixer with digital control. The analog signal paths meant less expense, but without sacrificing digital convenience. The result is the PGA2310 2×1 Mixer Project.

Components are mostly socketed, so despite the name, a PGA2311 volume control could be auditioned instead, or alternate op-amps selected in place of the OPA2134. TT P260 panel pots and KIL OEJ knobs, combined with a laser-cut hardwood exterior provide a professional-looking finish. And of course, a Teensy 3.2 reads the potentiometers, sets volume output values, and does all the mixing maths. Source code, schematics, and more can be found in the project thread.