All my replay videos had audio missing before the video ended like couple seconds or even from half the video to the end are completely silent. I'm facing this problem like month and have no clue whats wrong with the feature.
I assume this could be drivers problems since i had previous version installed that working flawlessly which i couldn't remember because i'm going stick with latest updated one. But when i use common recording feature i had zero issues from it.
Yet the problem still persist the only thing i could try but most of times didn't work are turning instant replay off, delete replays cache and turning it on again. Then the problem back again if i using instant replay for longer period time.