Ah, you're right, it doesn't say that. It is optional though (check the source ;), that's why it starts with a #, so the parser can tell if you specified it.

It's not optional if your path starts with a #. Similarly, the sounds are not optional if your path contains .wav, .none, or .def, but only because of the way it guesses whether or not you've put them in. (not the best of checks, but that's how it does it)