In #2127 (comment) Github user @rdagger reports the following problems:

I've been having problems with I2S on the Feather nRF52840 Express running CircuitPython 5.0.0-alpha.4-260-g2b02750f1 (2019-10-26.)
I'm using an Adafruit I2S 3W Class D Amplifier Breakout - MAX98357A (PRODUCT ID: 3006).
I've been using the example programs and here are some issues:

1. The sine waves only work well at certain frequencies. Some frequencies causes the Feather to reboot in safe mode. Other frequencies have very rough waves.
2. Often after an I2S program completes without errors, the Feather will reboot a few minutes later.
3. WAV files have to have a frequency of 22050Hz and 16 bits per sample or else they are garbled.

Look into these reported issues and fix them as necessary.

Note: I am self-assigning this so it stays on my radar, but other obligations will likely prevent me from doing more than the smallest amount of investigative work until at least the week of November 11.