From website:
The simplest one transistor audio mixer circuit diagram on the net. It has only one transistor and can support as many audio signal as you can afford. BC109C is a very cheap transistor and with a few resistors and capacitors you can build quickly a mixer circuit diagram that you can use in your pirate radio or wherever you want, imagination is the final frontier. Please keep in mind not to use higher voltages than 22V as a power supply for this audio mixer.
http://www.electroschematics.com/327/one-transistor-audio-mixer/
