Routing Audio Ground - needed or not?
It's been quite some time, so I'll go ahead and answer my own question in case any other nutjob with stupid questions comes through here:

YES, YOU NEED TO TIE ALL GROUNDS TOGETHER! 

I breadboarded the prototype and it all worked fantastic, without any hum whatsoever. Star grounding is a good idea to ensure you won't have any ground loops.

Also, I added an optocoupler into the design, to separate digital and analog sections. Will update answer after completing prototype.