You can try putting it in the 'sound' folder, but it won't make a difference.
You'll need to convert the .raw file into a .wav file and then place it in the 'engine' folder.
Once in game, press Shift+A, a load of sliders will appear on the right hand side, find the 'Dump Valve' sliders (about halfway down), click on the 'L' beside those sliders and load the dumpvalve.wav file.
Attached a screenshot just incase that wasn't too clear.