A: "Where you from?"
This joke is a pun. The word Liberian means someone from Liberia, however it sounds like the word Librarian, which means someone that works at a library.
B: "I'm Liberian"
A: "Oh, sorry!"
A(Whispering):"where you from?"
In the US, librarians are characterized as being very interested in keeping the people around them quiet. This is probably because you are not supposed to be very loud in libraries.
So, the reason why the person whispers the punchline is to show that instead of Liberian, they heard Librarian.