YouTube is only in stereo anyway
Why would they list 5.1 in the recommended youtube video settings then if youtube only supports stereoRecommended bit rates, codecs and resolutions, and more
Container: .mp4
No Edit Lists (or you may lose AV sync)
moov atom at the front of the file (Fast Start)
Audio Codec: AAC-LC
Channels: Stereo or Stereo + 5.1
Sample rate 96 khz or 48 khz