I know this is an older post, but I've been scratching my head for weeks trying to resolve this issue. The sound only (not video) would drop out intermittently for 1-3 seconds randomly, but more often during transitions between commercials and when returning to normal programming after commercial segments. I have a new Samsung QN95B Neo QLED TV and Samsung Q800 Soundbar with a Cox SCXI11BEI wifi cable box. All are connected by HDMI 2.1 cables. The TV detects and selects HDMI-eArc signaling between the devices as expected.
I finally landed on a setting in the TV sound options that has resolved the dropouts completely. In Samsung its under SOUND --> Expert Settings --> Auto Volume (On or Off). It was set to "On" when I had sound dropouts. I set it to "Off" and voila no more sound dropouts. If you have a Samsung TV, check it. If you have some other brand there may be a similar setting.
It seems that this setting, when On, causes the TV to try to detect volume differences and adjust them automatically. I assume this is to try to alleviate the volume difference between commercials and programs. Seems that in my case this setting has the side effect of causing the annoying dropouts.