Audio problem: When Buckwheat says, "It's worse than I thought", while watching Alfalfa and Darla in the boat, you can see Uh-Huh next to him mouthing that line.
Audio problem: At the talent show, where Waldo does his duet with Darla, you can tell he's lip synching. Sometimes he mouthes the words too late or too early. And in shots where he has his head up, his Adam's apple isn't moving, a sure sign no noise is coming out of him.
Audio problem: When Darla and Alfalfa are in the boat, Darla says "Oh Alfalfa, you're a sweety poo" or something, but when she says "sweety poo", her lips don't move.
Answer: I tried looking this up on the IMDb and couldn't find anything about Robin Williams there. My only suggestion is to watch it again and see if there is a list of extras in the credits somewhere. Maybe that's all the part was, an extra.