Creating custom soundscapes

From Valve Developer Community

Yes, you can. The SDK has been updated so that soundscapes are no longer hard coded to only reside in the soundscapes_manifest.txt script file. With proper naming of your soundscape script the game will load it when the map loads.

The soundscape name and location should be as following:

    <game directory>\scripts\soundscapes_yourmapname.txt

For example, with a Counter-Strike: Source map called de_kilo2.bsp:

    <game directory>\scripts\soundscapes_de_kilo2.txt