Why does Dr. Kennebrew Beauregard utter gibberish in the middle of his speech?

In-universe, we see him constructing his piece, psuedo-real-time from multiple takes [cut together for our convenience]. He periodically coughs, or corrects himself and starts that segment over.

The ma-ma-ma is an 'amateur' version of an actor or speaker's warm-up routine. You will often hear an actor 'making sure their voice is warm' just before a take. Coughs, throat clearing, loud expressive but nonsense sounds and facial stretches, to get them prepared for the scene. During a take, if a line is fluffed, sometimes they will just pause and pick up; other times they will joke or yell or swear or babble [searchYouTube for out-take or 'blooper' videos] depending on their frustration level. I'm guessing that time, our character's frustration was heightened.

Out-universe, it just makes him look more ridiculous - gives the audience good clues that the views expressed by this character are not necessarily going to be those of the director, or that the audience would agree with either. This is emphasised by the increasing use of colour in the projected videos as the scene progresses. We start in black & white, with the character making his case for his viewpoint; but each successive reference behind him gets more and more emphatic, bordering on the ludicrous. As, presumably, the end result would be in black & white for the TV, it makes little sense except to ramp up the emphasis to his outrageous views.

Had we seen his piece finalised and fully edited, it wouldn't have had the same effect on the audience. That we see him in the making of it, dents the character and makes him seem more fallible to the audience. It points us in the right direction for the plot, even if we had never seen a trailer, plot synopsis, or even knew who the director was.