Hi,
At the time when we developed this page model, we decided not to implement the autoplay feature since autoplay is not generally considered good practice and playback should be in the control of the end user.
If this is something that you need though then you can make the necessary changes in your own local install at:
/modules/xerte/parent_templates/Nottingham/models_html5/youtube.html
You will see that the iframe object supports autoplay but it is off by default. You can force it on always by changing the value at line 27 but it would be better if you added some code between lines 107-134 to parse the actual pasted embed code.
Regards,
John