Many audio mixer circuits have been published here. But I think, this one is the simplest of them. This very simple audio mixer circuit uses only one transistor. The base emitter junction of the transistor is biased by the diodes D1 and D2. The signals to be mixed are directly coupled to the base of Q1. Each input lines are current limited by using
a 33K resistor. With the used component values the collector current is around 1mA.