How to mix 8bit/8kHz unsigned linear PCM data in AppsScript?
Assuming that abyte[] and bbyte[] are two arrays with samples valued from 0-255, this is the solution:

combined = [];
for(i = 0; i &lt; abyte.length; i++)
{   
    sample =  2*(abyte[i] + bbyte[i]) - ((abyte[i]*bbyte[i])/128) - 256
    combined[i] = Math.round(sample);
}


I have a full example with both wav files extracted into arrays (I did this using matlab).  Also I verified that it plays back fine using matlab.

https://jsfiddle.net/4hsfo96g/