First time I've heard the Vulcan described as "silent". It's normally just about the loudest aircraft flying.
The wind can blow sound waves away and objects can block it .. maybe the wind was blowing away from the witness and there was buildings between it and him.
Talking of over-flights, I was once by the garden festival site on those new housing estates when a god awful racket drew closer. I looked heaven-wards to see 3 Shackletons flying
very low in a V formation making a noise like only 146 cylinders firing sequentially could.
When they were being withdrawn, did they do a tour of the UK? If so, I caught that as they flew towards Speke airport/Widnes.