Thanks: very interesting to know that. Given what WWI accounts say about shell-holes, often full of water or mud (people sometimes fell off the duckboards they put over them and drowned in the mud before they could be pulled out), and much used to take cover from enemy fire, you might have some slurpy, sloshy sounds and a lot of gunfire, I suppose.
Actually it seems to be true that "ambiance" does often mean "ambiance sonore". The Trésor has a nice quotation from Cocteau:
"1. CIN., RADIO et TÉLÉV.
a) "Atmosphère de la réalisation ou de la projection d'un film." (Giraud 1956).
Ambiance sonore. "Ensemble des bruitages créant l'illusion de la réalité." (Voyenne 1967) [...]
ce que les cinéastes appellent, dans l'atroce vocabulaire moderne « l'ambiance ». C'est le « mixage », le mélange des voix humaines, des cris d'animaux, du vent dans les feuilles, de la mer et de l'orchestre qui les accompagnent. J. Cocteau,
Le Foyer des artistes, 1947 p. 196."
http://www.cnrtl.fr/definition/ambiance