The other week me and my wife climbed into a Cwm/Corrie/Cirque in the lake district on the side of St Sunday Crag in the UK. I could distinctly hear someone talking. I could basically hear every word. I looked for this person only to realise they we're on the other side of the cwm. I checked my map, they we're about 0.5-1km away!
I was at X they we're at Y. Each Square on this map is 1km.
Whats the process that allowed me to hear this person over such a long distance? I've experienced this phenomenon several times. Always in Cwms.