Are you asking if running Windows 8 on hardware meant for Windows XP will be a problem?

I am inclinded to say yes, but need actual hardware specs to confirm.

Microsoft maintains a compatability mode list they place sites on.  You may be being placed into compat mode for no actual reason.  You could disable it and see what happens, but its often used to ensure the page loads properly.

In general I personally dislike auto-playing anything where you may not expect it.   The person may not have speakers on, may have speakers playing something, may not have speakers at all, etc.

I find it's a better user experience to have a button that is click to play, but you could either transcribe the open or similiar, with a eye catching font styling to draw attention to you want them to play it.

As added bonus, if you tied metrics to the play feature, you could actually see how many people bother to hit play.