I have some copies of UHD-BluRays on my NAS and a Gigabit Network!
Recently i copied the new "Top gun 2" Movie with MakeMKV to my NAS. (4k with english and german Dolby Atmos and subtitles; filesize approximately 80 gigabytes). If i play this movie on my Raspberry 4 (4 gigabyte), there are minimal Sound glitches (milliseconds of silence sometimes) during the movie. The picture is fine. The glitches go away if i disable pass-through and use 8 channel PCM. My reveiver shows that the movie is played with 24 pictures per second which seems to be the original quality on the UHD.
If i change the picture bitrate with Handbrake (H265 default UHD quality with audio passthrough) the filesize is dramatically smaller (1/4) and the raspberry plays the Athmos Audio passthrough without any glitches (i even cannot see a picture quality difference but it tooks very long to convert).
Is there a bytes per second limit that the Raspberry 4 can handle? For me it looks like the Athmos Audio-passthrough in combination with the high 4k picture quality exceeds this limit. I think its not a network problem which seems to be fast enough (no buffering message).