Question about Audio signal trace layout (TLV320AIC3204)

I assume all your high speed signals are LVDS.

If you are doing signal breakout from the top, then as long as you do not plough through with single ended signals in that area you should be fine.

If there is room on the bottom, that may be a preferable place (so you leave a solid plane). Just ensure you have decent separation rules.

That said, there is no general reason not to use a plane layer for a few single ended signals. Just make sure it is only a few so the copper balance in the stack is not out of whack. Your PCB vendor can help with this.

Give plane preference to the high speed signals, and assuming they are all on the bottom with a solid ground, you should not have any real difficulties as the LVDS signals will all be referenced to ground.

The mic area is probably isolated that way because it can have a very high output/input impedance, making it extremely sensitive to voltage noise. It looks like it has to traverse the whole circuit board (from edge to middle) without any buffering... this would make it extremely succeptible to both digital signals and audio crosstalk.

Additionally the headphone out can feed a relatively low impedance, so this could cause voltage noise across the ground plane. (ground bounce, ground loop)

This isolation scheme keeps this at bay by keeping both unwanted voltages and unwanted currents out of sensitive areas. As you point out, it may not be necessary depending on what you're doing and how you're doing it, but the dev board designer felt it was the best way to go for the demo.