Audio Mixer circuit diagram Using FET 2N3819
This simple circuit mixes two or more channels into one channel  stereo into mono.
Parts
R1, R3 10K Pot
R2, R4 100K 1/4 W
R5 6.8K 1/4 W
C1, C2, C3 0.1uF
Q1 2N3819 FET
MISC Wire, Shielded (Metal) Case, telephone Or Other Plug For Output