Apparently Sir Toby Belch has been snacking on pickled herrings while getting drunk. Maybe he has just belched and is putting the blame on the herrings. How now, sot! could either be a greeting to the Clown who is a "sot" in the sense of being a fool, or it could be addressed to himself, who is a "sot" in the sense of being a drunk.
Simon Armstrong as Sir Toby Belch RET, 2017