[quote="deadeye":j0tvtmcl]If you like, you can pm your .cap to me and I'll take a look. It might help to have another set of eyes on it.[/quote:j0tvtmcl]
I'm not sure that'll be so easy - it's nearly 20MB big, and very, very long. (And probably very silly.) But thank you very much for the offer. If I can't solve this tomorrow, I might send you something to have a look at.
However - it appears that despite my despair, at least one of these issues has a rational answer. I still can't explain the crashing, or why the damn thing was refusing to play anything via the channels, but I might be able to explain the tinny sound I kept getting at random. My computer has two sound drivers (and two sound cards) installed. I just ran the game on my wife's computer, and it seemed fine. Of course, when these things are so random it's hard to be sure, but it's a possibility. One that I will look into further tomorrow, when I've gotten some sleep.
By the way, many thanks to all of you for your advice and your patience. Another thing about Construct that is really awesome.