Quote: "I want to make metalic echo.How can I make metalic echo when I enter ventilation duct?"
A similar question was discussed
here where the OP of that thread wanted to have "snow sounds" when using a snow segment. As I pointed out, you could replace on of the current "materials" used in the "audiobank\materials" folder, or you can mod the source to add more materials. Replacing the sound means you find out which sound is being used and replace with the sound you want. However, note if the frequency is incorrect, it will not play.
Swapping/replacing sounds work only if you know how segments work and the "materialdefault.txt" file work together. You could end up messing up your materials sounds and have to reinstall.
The easiest way? Just use a trigger zone and have the sound play. The duct is metal, so that sound can stay. But you can have a "louder" metallic echo play via trigger.
There's no problem that can't be solved without applying a little scripting.