On the animated version of the Peanuts cartoon, all of the adults sound the same to the kids... an incoherent "Wah wah wah wah wah..." Imagine if your mom was droning on about why you didn't pick up your clothes...

I'm not sure how that line really fits into the story line of the song, though. If he says "You all sound like Peanuts' parents." then who exactly is he talking about? The other people in the hospital? Well he obviously understood the doctor at the beginning of the song... that why he starts telling the story. Is he talking about the aliens? Well assuming there is more than one alien around him... he was still obviously able to understand them because "ET revealed to me his singular purpose... He said 'You are the chosen one...'" So who exactly is it that he can't understand?

Anyway... maybe it is the correct line and I'm just overanalyzing.