Ambient Sounds - custom ambients
A description of how to use custom ambient sounds in your map is described in three places.
1. For the zoned approach. Read the 5th full paragraph in the "Speaker" section: Ambient_Sounds,_a_zone_approach#The_Speaker
2. Creating a custom soundshader: Ambient_Sounds_-_Zone_(using_triggers)#Tips_and_Issues,, under the heading's "Custom Soundshaders" and "Custom Ambients"
3. 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.