Is “whisper off” a noun here? Does it mean the same as whisper?

A something-off is a competition at the activity. You might have a cook-off or a dance-off. He's suggesting the way they are whispering their lines is an attempt to outdo each other, or gives that effect. It's often used in a jocular or mocking sense.

a "blank-off" is a way of saying a contest. So having a "whisper-off" is having a contest to see who can whisper the quietest.

I would take "whisper off" to be a tongue-in-cheek description of the quiet dialogue. It's playing with phrases like "play-off game," or "Great British Bake Off" where the "off" tells you it's a competition. It's not a standard expression, exactly; I had no idea what you were asking about from just the question. Seeing the video description ("TV and movies have gotten harder to hear!") gave me the necessary clue to understand the sound byte you provided.

Adding “-off” to basically any word/verb can be used to indicate a *competition*. Examples: Bake-off: a baking competition. Bounce-off: a bouncing competition of some sort. Paint-off: a painting competition. Cook-off: a cooking competition. So by using the term “whisper-off,” they are indicating that there is some sort of competition or battle between people who are *whispering* to each other, or using whispers to compete somehow.

Yes, it’s a noun. You can hear he says “**a** whisper-off”, and only nouns use articles. It means “a whisper competition”, and it should be written with a hyphen. You can use verb-off as way of expressing that something is a competition and the method of competing is the verb: sing-off -> a competition of singing bake-off -> a competition of baking ski-off -> a competition of skiing One things I will mention is that this does kind of imply it’s kind of impromptu and not very serious. So a “ski-off” could happen when you and your friend both think you’re a faster skier than the other, so you decide to have a “ski-off” to settle it. But you wouldn’t really call a professional race a “ski-off”.