

Well, the fastest, crudest mixer (aside from just physically connecting
all your inputs together directly) is to run each signal through a 
resistor to a common output.  The resistors give some isolation so the
signal sources aren't driving directly into each other. 

Taking a wild, seat-of-the-pants guess, I'd say that if you are mixing
high-impedance sources like some dynamic mikes, electric guitar pickups,
etc., your resistors ought to be maybe 5K - 10K or so.  If you are mixing
line-level stuff or the outputs of a Walkman, or such, I'd go with 1K.

input 1 --/\/\/\/ -------------+--- output
                               |
input 2 --/\/\/\/\-------------+
                               |
input 3 --/\/\/\/\-------------+

