Audio problem: When Lewis and Betty go into the moon walk to have sex, she tells Lewis (who she thinks is her boyfriend, Stan) to take off his mask and he shakes his head no. She then says, "Oh Stan, you're so kinky" - but her mouth never moves. She is simply smiling. (01:09:45)
Audio problem: When the nerds have their first party to impress the Tri-Lams, the first song that's playing when they are all sitting around does not match the equalizer on the stereo lighting wise.
Audio problem: At the start of the homecoming carnival, there are guys playing trumpets. They drop them as if they're done playing and the sound continues a bit longer than it should.
Answer: First, Lewis and Betty are not "having sex" - he is raping her, and the writer and director later described it as "not excusable" (https://decider.com/2019/07/26/revenge-of-the-nerds-rape-scene-regret/). Second, the song you hear is "Right Time for Love" by Jill Michaels and Pat Robinson. It was not written for the film.