Backchannels (listener responses) in Spanish @Joze: An onomatopoeia is a word that imitates a sound it's describing (like "meow"). Backchanneling is a separate concept (check the Wikipedia page I linked above).
Why is 'estar muerto' used instead of 'ser muerto'? One way of thinking about it is to see estar as not necessarily dealing with permanence, but condition (estado) as opposed to being (ser). Being dead is a (albeit permanent) condition or situation that a human is in.