I wasted most of my weekend free time on the audio out. The audio works using the userland procedure from Adafruit. I then tried and failed over the course of a few hours to do the same using devicetree overlays, but can't seem to get the same behavior overlaying the pin-function. I'll have to go digging through the driver.

Music sounds OK through my trusty old Altoids tin headphone amp, but there's a ton of noise floor coinciding with CPU load. It reminds me of my first PC sound card that telegraphed HDD and CPU activity through chirps and squeaks. Of course this is expected since the PWM audio power supply is the 3.3V IO rail on the Broadcom CPU. The A+ / B+ / 2 digitally buffer the PWM signals with a dedicated power supply, but adding that to this board is overkill I think.

I also tried out 62MHz mode, which the WiFi Pants board and one of my custom HAT boards didn't like. I did get it working with one of my other HATs, which was rigged for 1-bit mode and used a smaller series termination. I deviated from the schematic and used 150 ohms on the first Pants board to see if I could simplify the BOM, but that was probably a bad choice. I didn't have time to scope it, but given my previous testing I would expect to see some sad excuses for digital pulses.

As for the next rev, it took until now for me to realize that the serial / power / audio header blocks both USB connectors! With FTDI continuing to punish consumers maybe a different location and pinout is in order...