Simple additive mixers use can be analyzed using
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 is set to 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 Link

Create LM386 in MultiSIM BLUE Link 

Multisim schematics file download

Multisim schematics file with an audio output download

Used MultiSIM BLUE for the simulation