user11796: 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 )

user12143: 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.

user10554: 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. 

11796: <p>Youtube states that it only supports <a href="https://support.google.com/youtube/answer/1722171?hl=en">Stereo or Stereo + 5.1</a>. </p>

<p>How does this work? Does it mean that there should be two audio streams included in the container or is this a special encoding?</p>

<p>( I am using Premiere Pro )</p>
 12143: <p>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.</p>
 10554: <p>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.</p>

<pre><code>Recommended audio bitrates for uploads

Mono    128 kbps
Stereo  384 kbps
5.1     512 kbps
</code></pre>

<p>YouTube doesn't currently support 5.1 playback. If you upload a 5.1 audio track it gets <a href="https://productforums.google.com/forum/#!topicsearchin/youtube/5.1">converted to stereo</a>. </p>
