Its for this incredible little Arduino based synth - all of the sound is coming from a single 8 bit PWM Channel -
It works will through an audio amp, but is very quiet through the LM386 - unless I just output one or two channels.
I assume its down to two things -
1) The LM386 is seeing the underling 5V square wave and adjusting its gain accordingly
2) The code outputs the four audio channels through on PWM channel by averaging them and modulating the duty cycle, this has the effect of bringing all four closer to the longer average which is 0 (50% duty cycle).
Commercial audio amps seem to be able to work with this, I assume that they are more effective at filtering out the underlying square waves, but the LM386 is very effective with one voice - pretty good with two voices - hopeless with four.
If your interested I plan to launch the code in the next few days, its based on a synth which has been around for a while but which was not documented so not widely known or used. I have refactored it into two classes, optimized it and documented it with a plan of launching it with a couple of example projects to demonstrate its incredible power - its entire sound comes from varying the duty cycle of one 8 bit pwm output.