Translation:She did not hear what you spoke yesterday.
My translation was also accepted by DL: "She did not hear what you spoke ABOUT yesterday."
She hasn't heard what you said yesterday ....... or.... She didn't hear what you said yesterday........ both make perfect sense; unlike the supposedly correct answer on offer above!
"Voi" is not really necessary indeed once you say "ați vorbit". "Despre ce ați vorbit" would refer to the content of the conversation while "ce ați vorbit " is the conversation itself