What to call someone who (silently, but loud enough that the talking person hears it) cries out "wow" (throughout another person's monologue) all the time as an expression of delight or admiration (?) The talking person is someone of authority.
They sound like they are a sycophant.
Some slang terms:
They could also be a brown-nose / brown-noser. That's pretty vulgar. Their nose is brown because it is firmly wedged in someone else's ass crack.
Or they could be a suck-up. They are sucking up. That's also vulgar.
I would like to suggest 'wowser', but that Australian English word already means about the complete opposite (more information here and scroll down a bit).
Considering your response to my comment, the context seems of a person who unnecessarily praises (or simply agrees) with exclamation or surprise to the person who's an authority.
I'm not sure there's a term that represents a person calling out wow every time (and in this case, you may simply go with your own version) but there's a good term for a person who does it just to praise falsely. This often refers to a person who falsely praises his/her boss by simply agreeing to whatever the boss says.
One of the terms is flatterer
I consider that in this context, wow is an expression of surprised admiration.