So good news, bad news and some more good news. Oh, then some more bad news.
I've populated the new board (good news) starting with the power components. One of the changes I made was to make space for a much larger heatsink for the LM7805 5V linear regulator - the input voltage is 12V from the transformer and it was getting very hot. A much larger heatsink is keeping the LDO cool enough to not constantly go into thermal shutdown, which is nice. However, I read the temperature with a IR thermometer - the heatsink is around 85 degrees (celcius) so the internal temperature must be through the roof - and while it's not in thermal shutdown, simply putting the lid on might make that happen. Nonetheless, I'm counting this as good news because otherwise I don't have much good news...
That said, the new microSD card reading is working perfectly now that the power lines are wired correctly. I can now read files from the SD card - definitely good news.
The Bad News
I added some headers for the DAC and plugged it in, hoping for glorious sound from the little PCM5102A module I bought from eBay/Amazon (don't remember which). It has a headphone socket on it, so I don't need to populate the amplifier stage to hear audio. No sound. This is annoying. Then I noticed there's no red light on (indicates power) so went to investigate. The DAC has two rows of pins adjacent to one another. One side is just fine, one side is... well... backward - and there's absolutely nothing I can do about it. I've used dupont cables to plug the DAC module in, but that's definitely not a long term solution. Still, with the pins round the right way I can hear audio... woohoo! Now on with the amplifier.
More Good News
I had a couple of "hiccups" (including a short under the ESP32 that caused me some confusion when a simple pin state change took down the entire board by connecting GND and 3.3V. After I fixed that, I heard sound from the speakers! I was a little worried about the amplifier stage - I've never built one and my prototype didn't work - but it works perfectly! All I need to do is fix the header issue and it looks like the main board is done!
Don't Get Your Hopes Up
Fortunately (depending on how you look at these things) I decided to put the lid on the thing before I ordered the new PCBs with the corrected header. I'm so glad I did - the lid doesn't fit (not even a little bit). The underside of the lid which houses the speakers is a very odd shape underneath. There's a few things in the way (a bulk capacitor, both LDOs, the DAC) that stop the lid from fitting on. I thought I'd measured this... but apparently not. So now I need redesign the PCB to cram anything over 12mm into the top right corner and work out how to cool down that LM7085...