As James Jenkins pointed out in his answer, http://discuss.area51.stackexchange.com/q/22863/116910 clearly established that What-If questions are not, in general, what Worldbuilding Stack Exchange wants. As a moderator there, I cannot emphasize this enough. I'd recommend looking through the following meta discussions (and links therein):
We haven't specifically banned What-If questions - there's no custom close reason that we use, although once upon a time we threw around ones to combat related problems - but the consensus seems to be that these questions are off-topic for Worldbuilding, in the majority of cases.
Why? Well, the questions are often too broad. I don't have any statistics to back this up, but I'm guessing that we're one of the Stack Exchange sites with the broadest questions. It's different from Stack Overflow, say, where there's often only one, two or a few solutions to a problem. On Worldbuilding, we average over 4.5 answers per question. That doesn't mean that all of our questions are overly broad, but some are, and What-If questions often fall into this category.
This is something that worries me about Speculative Science, and what should have worried me about What if? Even on Worldbuilding, I typically manage to cite pretty good scientific sources on most of my answers to science-y questions, because they usually rely on the laws of science in our universe. If you ask, "Is this system of moons stable?", I can give you a good answer based on orbital resonances to figure out if the two orbits will destabilize one another (I know, the three-body problem is hard, but you can make some basic estimates in certain scenarios). I know papers to cite and mathematical expansions that can be directly applied.
But on a question that involves new laws of physics, what do I cite? What do I look to? Do I formulate totally new equations? There's a lot more speculating that has to be done, and it's crucial that answers are more like educated guesses. You must have answerers who really know what they're talking about, not just folks who can only take shots in the dark (and I know that though I pride myself on my science knowledge, I'm more in the latter group). I foresee that a community of experts is going to be needed here, potentially more than on any other Stack Exchange site. Will that happen? I don't know.
This segues into my other answer to your suggestion that these questions could go to the pure science sites. This is - if I may be blunt - a terrible idea. We've experienced this on Worldbuilding when migrating questions to and from the science sites. They didn't want some questions because they were overly speculative; we didn't want some questions because they were just What-Ifs with no tie to Worldbuilding.
Again, James Jenkins has a good meta citation, but I'll address one specific example: The question about sound moving faster than light. It's already been asked and closed as off-topic on Physics Stack Exchange. For lack of a better close reason, the non-mainstream science reason was used, but Ben Crowell's comment captured the sentiment that Physics Stack Exchange (and myself) feel is sometimes applicable:
There is no way to answer a question about what laws of physics could exist in alternate universes.
Well, it's not always impossible, but it's way too speculative for the pure science Stack Exchange sites.