Hello,
it would work with 20 MHz, but not better regarding stereo playback at full rate, because this is just a matter of free cycles. But with 20 MHz fullrate will be 78kHz, so just a little better. I think the next improvement I will make is to use a 256R-R network to go from 8 bit to 16 bit with just another port and 3 resistors.
I don't find the circuit for the audio output. Increasing bit resolution is always about decreasing PWM frequency, the only way around is to use either a AVR with high-speed PWM (like Elm-Chan did with ATtiny85) or combining two PWM outputs.