Distorted / buffer overrun / clipping audio problem

[cementimental]

Hi, discovered espsynth86 recently, very excited to use it as I'd been looking for something similar to the Teensy audio library but which runs on still-easily-affordable board. :) After some messing about I figured out how to get it compiling and uploading fine via Arduino IDE.

I'm using a NodeMCU 1.0 board in PDM mode and it's working but I'm getting extreme tearing distortion whenever any sound goes over a certain level. E.G. even a simple test script with 2 sine waves causes it. If I add a pot-controlled VCA it seems like the distortion starts abruptly as soon as the volume exceeds 50%.

It sounds to me more like buffer overrun or something than simple clipping; really extreme and on the waveform it appears as loud spikes rather than clipped-off wave.

As well as my own tests, the examples and the synth file shared in the comments on this video all sound extremely distorted, completely unlike the clean sound in the video: https://www.youtube.com/watch?v=uRPdX0R1gKY

Tried some different sampling frequencies and bitrates in the example .ino code but that doesn't seem to help.

Also tried it on a Wemos D1 Mini board with the same issue.

Any ideas would be amazing thanks!