Dean is/was mute due to abuse maybe(?). He then meets Castiel and I believe they begin dating and he says his first words (again) to Cas. Cas possibly encourages Dean to speak at his own pace. Sam and Jess do not know that Dean has began speaking.
Sam does not know that Dean has began speaking.
Dean spills coffee on himself and then says the first words Sam ever hears from him.
Fuck. Sam is grows angry and starts yelling at Dean because it is the first word he says. Dean begins to have a flashback and then speaks multiple words seeing John instead of Sam. Cas grows angry. Sam feels guilty and explains himself.
I don't remember much more though. :/ Just that specific passage.