It seems that the availability of the mono output option is somewhat dependent on whether your audio device/drivers support it. Perhaps you could try updating the drivers if newer ones are available.
One possible workaround is to use VLC (a free media player) to play the videos. Tested successfully with Version 2.0.5. Once you have downloaded VLC, choose Media -> Open Network Stream..., enter the YouTube address and click Play.
- your example URL will not work unless you remove the feature=player_embedded part.
- VLC tries to play the stream at highest quality. For lower quality (but faster streaming in some areas) add "&fmt=18" to the url. For example, http://www.youtube.com/watch?v=poiFKnVwMeQ&fmt=18
- (videos sound much better thru VLC than any browser!)
Then, on the Menu window, see if it is possible to choose Audio -> Audio Device -> Mono.
If Mono is not listed then go to Tools -> Preferences -> Audio and set Output module to DirectX output module. Then click Show settings -> All, click Audio -> Output modules -> DirectX and see if you can choose Mono for Speaker configuration. You might need to restart VLC for any changes to have an effect.
The advantage of using VLC is that normal stereo files still play fine in other programs.
Please report back to let us know if this works or not.