UPDATE: Some .mkv 4K YouTube videos still can be converted but others can't

For example, when https://www.youtube.com/results?sp=EgJwAUgo6gMA is downloaded from YouTube using the 4K Video Downloader in .mkv UHD 3840x2160 (4K), there is no audio. When the no-audio .mkv 4K is converted into .mp4 4K, there is still no audio.
However, other YouTube videos can still be downloaded in .mkv 4K with both picture and sound.
For example, when https://www.youtube.com/watch?v=VzgpJ3mD_Kk is downloaded from YouTube using the 4K Video Downloader in .mkv UHD 3840x2160 (4K), there is both picture and sound.
Conversion from .mkv to .mp4 can still be accomplished successfully for that video.