I’m having a similar issue with a DIY Norns shield. Certain apps or sounds within apps have what sounds like audio clipping. e.g. Dronecaster does it very consistently with Nautilus but doesn’t do it at all with Sine or Supersaw. I’ve heard it in Timber Player when I play more than about three voices and in Takt on almost any sample I’ve tried. Most apps never do it. It suggests to me that values are overflowing when signals are added together. It really doesn’t seem like a hardware issue because most apps do not have the problem.
I’ve fiddled with all the level settings, reimaged my SD card twice now and the problem persists. Kind of annoying - some interesting apps are unusable because the sound is very distorted and clipped.
Any suggestions of what to try?