I have now received the boards and assembled one device.
I connected it to my SAMD51 Breakout Board running CircuitPython and tried to play a wav file, with no luck.
I couldn't understand why wouldn't it work, it's such a simple design, I suspected the jumper wires. In the end it took me some time, but I realized that my SAMD51 board doesn't have the analog reference connected!
Luckily, that pin doubles as a GPIO, so I had it broken out. I connected 3.3V on it and got reliable and fairly clean audio! The only problem is that it is a bit quiet.
I am using a 5mm buzzer here, I compared it to the 7.5mm one used in the Circuit Playground Express and that one is a lot louder. I might make another revision with a larger speaker footprint. But first I might test it out with an external speaker.