Ambient Sounds - custom ambients
From The DarkMod Wiki
A description of how to use custom ambient sounds in your map is described in two places.
1. For the Zoned Approach: Ambient_Sounds,_a_zone_approach#Tips_and_Issues, under the heading's "Custom Soundshaders" and "Custom Ambients"
2. Using Ordinary Speakers: Adding_ambient_Sounds_to_your_Map#Custom_Ambients
Note that custom sounds need to be in .ogg or .wav format. See Sound File Formats for more detail on that.