Simple additive mixers use Kirchhoff's circuit laws to add the currents of two or more signals together. When two different signals are applied to an input of an amplifier (assuming the amplifier input impedance is much higher than the source impedances of the two signal generators) they will add At Vin Junction point. Nodal analysis at Vin point gives:

The following signal will be seen by the input of the amplifier:

Amplifier with Gain = 200

Below is the output waveform.

The following image is taken from o-scope simulation in MultisimBLUE

The simulated sound can be played on a speaker. Double click on the speaker symbol.

Use "Multisim schematics file with audio output" example below. Run the simulation for a few seconds, then press "Play sound button" and you should hear the mixed simulated sound.

Used useful links:

LM386 simulation model: http://ecee.colorado.edu/~mathys/ecen1400/Software/LM386.cir

Create LM386 in MultiSIM BLUE: http://ecee.colorado.edu/~mathys/ecen1400/Software/CreateLM386.html

Multisim schematics file.

Multisim schematics file with an audio output.

Used MultiSIM BLUE for simulation.