As a former serving infantry grunt (first gulf & bosnia only, QRL recce 102striker commander) I can assure you that gunfire and large calibre guns going off is fairly normal and the sound does carry over flat areas. You get immune to noises quite quickly as you realise that life goes on, you are no longer on duty and you need to deal with your normal every day things. Unless you are under contact (in which case the only people you are talking/listening to is your platoon net on your left ear and company net on your right ear) then believe you me you start to do things people wouldn't believe - because you only have 4 hours off and need sleep too.
Ive seen church masses taking place in the open whilst arty is firing away in the background, people playing basketball whilst mortars land 1km away, ive slept in countless places with allsorts happening (I did call home once to assure the girlfriend I wasn't dead but it only made things worse as SHE thought the "normal" noise in the background was worse than it was). So whilst the geezer on the line might not have ACTUALLY been under contact, it isn't surprising that to an average joe listening to the sounds of an active zone might think they were.