What's the deal with the flies?

Watching the movie it seems that there were lots of flies at the beach when they were shooting the movie. Kinda hard for the actors to not swat the flies when shooting a scene, so you see the actors swatting the damn flies all the time.

But why did the producers/director decide to include a louder sound effect for the flies? Really, sounds like the flies are on the mic.

Or... I just had this idea when writing this post. Maybe the flies are actual flies that were on their real corpses outside devil's world.