I've got the headphone jack and audio in place. I'll update the instructions to make it simpler to setup.
Theres an intermittent high pitched whine and some faint clicking/crackling when nothing is playing, if you play something from one channel the other has lots of white noise.
WiFi (and possibly SD) access causes extra electonical noise.
Its good enough for playing sounds through a crappy speaker in a project, or for simple radio streaming (you don't hear the noise over the audio) but not for any kind of serious listening or for headphones.
I have filters the same as on the Pi 1, and I think my routing is OK. So the only thing I can think of is that the routing of GPIO12/GPIO13 on the Pi zero itself causes it to pick up noise. Or perhaps I need better filtering on GND?
I've tried it on a board with the 5v regulator populated too.
(I did populate all the audio filter passives)
On the positive side I routed the right and left channels the right way around, which I'm happy about as I was worried I'd mixed them up!