This circuit is a simple mixer circuit. It can mix two signal channels and one channel is output. Using a codec circuit,Convert stereo audio to mono audio time.
-In circuit a FET number 2N3819 is main, it is better than a common transistors that a high impedane input and high gain too. so cause low noise and.. It can increase the number of channels too, By adding a VR1, R1 and C1 to the amount needed. Then connected to Buffett a new one. Most importantly is eating circuit current is very low.Can use with 9-volt batteries immediately.
When entering voice signal, one of input 1 and input 2. Audio is via C1 and C2 of each channel,served coupling signals to VR1 and VR2. To adjust the audio to the Fet Q1.Which it serves, including audio. Then expand signal the output pin S through C3 For coupling signal again, before leaving to the output.

This is the idea of the simple audio mixer circuit. Perhaps you might need to take advantage of it.

Some time you want high gain than FET, I think an OP-AMP IC is good way for you. The LF353 has JFET within it so very well same common FET but it has many components inside it, makes high gain and better than 2N3819 but in normal using we may need it because too cost in some places.

This is a very simple 4-channel audio mixer. We used a number IC of LF353 is the main equipment, the function of each channel combined with upper signal level. They is low noise for J-fet op-amp IC.