Sure, but you're not talking about a weird noise here that might be a cat knocking over a vase in the next room or something. You're talking about having all sound stopped around you. This isn't a normal occurance, and they could safely assume something is very, very wrong.
Think about it this way: if you were in a museum, guarding something, and suddenly all the lights went out, wouldn't you immediately raise an alarm of some sort? Doesn't matter that you don't know why the lights went out. It's a major occurance that doesn't normally happen.