The env_sound entity is a trigger of sorts that alters the processing done on sounds heard by the player. For example, the env_sound can alter sounds to have a strong echo as though the player is in a large enclosed area. This entity is used to assist in creating a particular atmosphere in an area.
Radius (radius) - Sets the radius around the area to use as the trigger area. When the player enters this radius, the sound style will be turned on until it is altered by another env_sound.
Room Type (roomtype) - Sets which sound style to use.
Normal (off) (0) - The default, echo-less sound style.
Generic (1) - A slightly more closed in sound than default.
Metal Small (2) - Quite similar to Generic, with slightly more ring.
Metal Medium (3) - As above, but with slightly longer echo.
Metal Large (4) - As above, but with longer echo.
Tunnel Small (5) - A drawn out, tinny sound.
Tunnel Medium (6) - As above, by with more drawn out echo.
Tunnel Large (7) - As above, but with a very drawn out echo.
Chamber Small (8) - Similar to Generic, but with more echo.
Chamber Medium (9) - As above, but with slightly longer echo.
Chamber Large (10) - As above, but with a long echo.
Bright Small (11) - Very similar to Generic.
Bright Medium (12) - As above, but more open-sounding.
Bright Large (13) - As above, but more open-sounding.
Water 1 (14) - A claustrophobic, muffled sound.
Water 2 (15) - As above, but with an echo.
Water 3 (16) - As above, but with a longer, ringing echo.
Concrete Small (17) - Similar to Generic, but with a short echo.
Concrete Medium (18) - As above, but with a longer echo.
Concrete Large (19) - As above, but with a longer echo.
Big 1 (20) - An open sound with a spaced out, ringing echo.
Big 2 (21) - As above, but with a longer-lingering echo.
Big 3 (22) - As above, but with a much longer-lingering echo.
Cavern Small (23) - A closed in sound with a fast-ringing echo.
Cavern Medium (24) - As above, but with a longer-lingering echo.
Cavern Large (25) - As above, but with a much longer-lingering echo.
Weirdo 1 (26) - Similar to Generic, but with a sharper sound.
Weirdo 2 (27) - As above, but with a high, ringing echo.
Weirdo 3 (28) - As above, but with a strange, high-pitched echo.
Don't forget that the sound style, once applied, is permanent until another env_sound changes it.
The above descriptions of the sound styles are at best a rough guide. To find the right sound for your area, you may have to experiment yourself.