Here is a belated response to DCA - gasman.
This solution won't work, unless I'm misunderstanding. I wanted (like the original poster, I think) to send HDMI out from a computer to a home theater system with a passthrough to my projector. But when I plug in my projector, which has no sound input, the computer stops outputting to HDMI at all in my case, and ouputs through the built-in computer speakers. So switching default audio to the built-in speakers will do exactly the opposite of what I want in this case.
I solved the problem with an HDMI splitter box. I tried two: one from Radio Shack which cost a fortune and a cheap one from an Amazon.com seller. Unfortunately, the cheap one rendered everything in green. The expensive one from RS did the trick, though, so I kept it. In case other people are looking, the model number was 1500474, and it ran $60. Here is a link to a description: http
/www.radioshack.com/radioshack-3-in-2-out-hdmi-matrix-selector/1500474.html#q=hdmi%2Bsplitter&start=8.