can you test with software decoding? One issue found on RPi4 is that HW decoding and audio passthrough don't play well along.
Interesting about the RPI4's HW decoding and passthrough - puts me off from getting one for a while as ideally I want HW decoding on all streams and passthrough of all formats.
Anyway, I tried the OPI-PC with hardware decoding off - unfortunately no difference to the audio problem. Played a stream where Codec info shows ff-h264(SW) as the video decoder, and with the audio format showing as RAW, RAW, RAW, RAW, RAW, RAW, RAW, RAW pt-dtshd, 8 bits, 48000KHz, audio still drops in and out. Only difference is higher CPU and slightly jerky video. Tried some other streams (AC3, DTS, TrueHD), same result. Tried the same AC3 and DTS streams from the RPI3, which worked fine.
Edit: Got it working after a bit of further testing. If Kodi display resolution is set to 1360x768 (my HD TV's native resolution), audio drops in and out whether passthrough is enabled or not. With Kodi resolution set to 1080P, audio passthrough for all formats seems to work fine. I haven't tested this robustly yet, but setting the Kodi resolution to 1080P seems to fix the dropout problem. No idea why and i don't even know why I tried this, but I'm glad I did. Will test further over the next few days.