This simple circuit mixes two or more channels into one channel (eg. stereo into mono). The circuit can mix as many or as few channels as you like and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want by just duplicating the "sections" which are clearly visible on the schematic.

2x 10K Pot (R1, R3)
2x 100K 1/4 W Resistor (R2, R4)
1x 6.8K 1/4 W Resistor (R5)
3x 0.1uF Capacitor (C1, C2, C3)
1x 2N3819 Junction FET (Q1)
1x Wire, Shielded (Metal) Case, Phono Or Other Plug For Output (MISC)