"You talked a lot."
Grammatically speaking, the "a lot" part is viewed as the object of the verb "talked". Objects in Hungarian take the accusative case, which is marked with the -t, and a linking vowel where needed. So sok ("a lot" as the subject of a verb) becomes sokat ("a lot", as the object of a verb).
Sok isn't an adverb here, it's a noun (or at least something very similar). "A lot" is what's being talked.
Adverbs, on the other hand, do generally not change in Hungarian. Since they directly modify other words, they cannot stand by themselves and thus get no case endings. "Nagyon köszönöm."
I've been away from working on Hungarian for a while, so I'm a bit rusty, but for this I wrote "Sokan beszéltél." I forgot that in this context, "sok" is considered a noun. It's hard to wrap my brain around the concept of "talking" as something you do to something. :) I think "talk" is intransitive in English in most common contexts. (There are phrases like "I talked him down," which means "I calmed him down by being rational or reassuring," but even in that case, I would consider the verb to be "talk down.")