Youtube states that it only supports Stereo or Stereo + 5.1.
How does this work? Does it mean that there should be two audio streams included in the container or is this a special encoding?
( I am using Premiere Pro )
It's not very clear what they mean. If you look further down on the same page it says it accepts 5.1 @ 512 kbps.
Recommended audio bitrates for uploads Mono 128 kbps Stereo 384 kbps 5.1 512 kbps
YouTube doesn't currently support 5.1 playback. If you upload a 5.1 audio track it gets converted to stereo.
The two major platforms that YouTube video players use are Adobe Flash, and Adobe Air. Flash 11 only supports 2 channel audio. Adobe air 3 supports multichannel audio. Also HTML5 supports multi channel audio. most multichannel streams will be PCM no bit stream, as you need licensing from Dolby or DTS to encode/decode. Furthermore multi channel audio is only supported for TV devices, on all other devices hasMultiChannelAudio() always returns false. This is probably due to HDCP / DRM requirements by publishers. Finally many smart TVs, especially Samsungs, will only send 2 channel audio from the optical output (again HDCP/DRM reasons). So you need to be sure what player / equipment you are using. A smart TV using ARC (audio return channel) to a receiver will have multichannel YouTube audio.