One of my biggest user complaints used to be that some of the videos were silent. After examining closer it seems they had MP3 or AC3 tracks? I didn't want to risk converting to any other audio formats so I'm not sure what else works, but "AAC" always works for my Roku 3 and TV.
I wouldn't want to wish this **bleep** on anybody but what if somebody converted hundreds of their videos to use an unsupported audio format and then uploaded them all to an S3 account. Talk about a nightmare. Maybe Roku can quit trying to fix things that aren't broken and focus on the video player features, like a VR 360 mode!! :) :)