Even sounds of birds would help. I believe audio plays a huge part in environmental immersion.
That's a fact.
-> background music, walking around... tralalalala, I'm in a swamp, Ohh there's a dead body over there.
or
-> background music, walking around... suddenly the music stops, I'm in a swamp, I only hear toads moaning, the wind is rustling through the leafs and a vague buzzing sound in the distance... as a proceed the buzzing sound becomes stronger... it's swarm of flies and OH MY GOD THERE'S A DEAD BODY OVER THERE!