It is customary to hold on the last syllable at the end of a song. At the end of the "Thneedville" song, what word do they wind up on? The town's name, causing the chorus to make a raspberry in unison!
"And then some other little critter's gonna come and make furniture out of our bones. And sit on it and have lovely dinners."
Although a bit of a tearjerker as well, when the animals are building a circle around the cut down tree, at one point the largest barb-a-loot is holding a tiny little stone, while a humming fish is rolling along a stone bigger than it and clearly expresses his preference that they switch!
The river scene and the dialogue between young!Once-ler and the Lorax after that scene.
Once-ler: I was heading into the light and you pulled me right back, and here I am! (hugs Lorax) You saved my life!
Lorax: Yeah, well, you know, not that big a deal.
Once-ler: It is a big deal! Look, I almost went over that waterfall! (realizes something) Wait...On my bed. How did my bed get in the river?
"OK, I've got a big day tomorrow, and I'm gonna get some sleep..." (walks off screen. Beat, then walks back on screen slumped over) "...Right after I find my bed."
"Oh, yes, right after the musical number about the kid who kept interrupting the story and was never heard from again."
The air-delivery guy. He's a big guy with a high voice and gets a featured solo in both the opening and closing song. And he ONLY sings!
O'Hare joins in the final musical number, seemingly about to change his ways....then says "Naaaah!" and starts singing "Let it die, let it die!" A Flat Character he may be, but at least a good joke can be made of that fact.
When Ted's grandma tells him about the Once-ler. The fact that she can appear anywhere is funny itself.
This line from the Cut Song, "Nobody needs a Thneed":
"The thneed is dumb./ The thneed is lame./ Who's the idiot who came up with that name?!/ *beat* /Alright, it was me."
The opening with the two minions.
The commercial for O'Hare involving bottled air. "Please breathe responsibly."
A background funny moment in the opening song, during the lyrics "We thank the lord for all we got." there's an small man wearing an umbrella hat holding a foldup tanning sheet. He's clearly looking at the two women on either side of him and once it blurs on him he backs up and his eyes shift from woman to woman.
Just before the Once-ler hits it big:
Lorax:"I'm playing poker, he's playing go-fish, and I think he's hungry." *cue duck eating one of the fish's cards*
Fish: Awww (in the most ridiculously cute voice imaginable)
Audrey tells Ted that if a guy were to show her a real tree, she would "probably marry him on the spot." The line itself isn't funny, but it is you realize that her voice actress, with her history with guys, had to say this line.