Rather than a second A/D converter chip for the mic I was also going to look at using a PWM ramp feeding one input of a comparator and the mic signal the other. It may have no advantage over a single chip A/D, given the low cost of these nowadays, but a fun exercise anyway.
Have you examined the material at this link?
<http://www.microtelecom.it/digimit/techart.htm> 73, Lyle KK7P
