We'll Meet Again, by Vera Lynn
This is, I believe, the second recording. The original was her singing solo. This version was recorded in concert, before a hall of young Royal Army troops soon to be on their way to join Montgomery's forces in North Africa. And they're invited to join her in the vocal about halfway thru.
The atmospherics are amazing. It is clear that she is singing to them!
We'll Meet Again--Vera Lynn (1940)