Given two candidates (for a C programming position), I would of course prefer those who know the syntax. Both because that's one less thing they need to learn before they can become productive, and because knowing C's syntax is indicative of spending a lot of time programming in C.

So to return to your comparison, if I were hiring for a position that required working math where the speed of sound was required... I would treat knowing that constant by heart indicative of their knowing the rest of that subject matter by heart. I'd want to test a lot of that as well. "What is the speed of sound?" would be like a fizzbuzz.