they're not the most sociable of people,and they must know it barks so much and are not trying to do anything about.It sounds as though it may be an uphill battle for you, if they are aware that the dog is barking then they may not take too well to being told to walk the dog and this could cause more conflict for you as a neighbour. Apart from leaving the dog, are you aware if they are treating it well, re feeding etc and discipline. If you suspect that it is mistreated you may have to go down a different path than if it is just ignorance into the needs of a dog.
Perhaps you could start a friendly conversation like' what dog have you got' - 'Isn't he beautiful' - 'Where do you walk him, perhaps we could walk together sometime' - along these lines as opposed to complaining. If you could become more of a friend then maybe you could gradually teach them a thing or two about training and dogs in general, sometimes this approach works better than confrontation which will alienate them, won't help the dog and you will have an enemy for a neighbour.