When you are speaking with a hearing person, you would never plug in earphones or stick your fingers in your ears. Very impolite!
Deaf people use their eyes for communication. For them, it is very impolite NOT to watch them, when they are signing or speaking to you. Don't look away to admire the surroundings, or to wave at a friend. Don't look at your phone or iPad, don't look at your notes or start writing them. The deaf person will assume that you have stopped listening, even though you can still hear the interpreter.
When you HAVE to respond to some auditory signal - a fire alarm, a doorbell, someone calling your name - always explain to the deaf person why you are looking over your shoulder or walking away: "Sorry, fire alarm! We've got to get out of the building!"