Hi…I also did a WM8731 audio board for the Teensy. Mine is significantly larger because I added a lot more functionality including a preamp (to support instrument/mic level inputs), MIDI and connectors. Regarding noise, it all depends on how good a quality you want. My board was specifically with guitar in mind which is very sensitive to noise and sound quality. For lowest noise, you need the analog 3.3V to be supplied by a dedicated LDO. You also need to do a fair bit of careful PCB design to keep digital noise from coupling into the analog ground.