This video demonstrates some simple sound dampening added recently. Any sounds that occur out of the player’s direct line of sight is attenuated and dampened. Listen for yourself:

The implementation is quite simple. The line of sight routine is already available and used in a few places already - AI, even the editor for drawing lines - and dampening the high frequencies is very easy: just apply a moving average on all the audio samples. I encourage anyone using SDL_mixer to try this technique out!