> My previous work with PWM relied on being able to set a hardware timer to control when the speaker turns on and off.

I've got an Arduino project where I did that. The output is on a single digital pin, audio encoded as PWM. It turns the output off at the beginning of an audio frame, and sets a timer to trigger an interrupt when it's time to turn the output on.

I've got a memory block of event structs, each event representing a write to NES sound hardware. I emulate enough of the features to support everything that the Legend of Zelda title theme uses. 16MHz is more than enough to decide on the sample for 2 square channels, 1 triangle, and 1 noise, outputting a 6-bit output sample at a 31250Hz sample rate. The output is only slightly scratchy; the polyphony is actually surprisingly smooth.