Is "observant" exclusive to vision?

The word "observant" means "watchful" or "perceptive":

A particularly observant child, he noticed even the slightest changes in the classroom.

is the example given by M-W. Specifically excepting the definition of "observant" that relates to following customs, is "observant" strictly visual?

I ask because I was having a couple of pints with a friend of mine who is blind. We were discussing expressions related to vision that people use without thinking, and then apologize to him for. For instance, "Can you look over this paper for me?"

He said that these expressions didn't bother him in the least, and thought it was amusing that people would trip themselves up over it. When I commented on his "observations", he remarked that that didn't bother him either, and further indicated that a person cannot be observant without vision. An increasingly drunken discussion of the origins and meaning of "observe" ensued, and now I have no idea which side either of us ended up on.

I know that "perceptive" is one definition of "observant", so I suppose it may generalize to all senses. If that is the case, are there specific words to describe being (the equivalent of) observant in other modalities (hearing, touch, taste, etc.)?

EDIT: I should emphasize that of central importance in the debate was the origin of the word itself: "ob" meaning over and "servare" meaning to watch, i.e., "to watch over". This origin ought to imply that observe at its root is related to vision, but the counter-claim to that argument is that "servare" itself is derived from PIE "ser" meaning to protect, which would not necessarily have anything to do with vision.

So if we're in agreement that observe can be used for the other senses, has anyone found other words to describe "observant hearing" (for instance)? Maybe derived from "to listen over" or something like that?

As indicated by the dictionary link that you mentioned, the definition of observant doesn't refer specifically to sight. And in this case the dictionary definition accords with common usage: it's acceptable and straightforward to refer to someone as being "observant" in senses other than sight.

As others noted, the dictionary definition doesn't require sight. It's also common to refer to "obervant $religionists" (e.g. observant Jews, observant Catholics), and to talk about observing protocols. In both of these cases "observe" means more like "keep" or "heed" and involves behavior, not just perception.