That sound is an internal ArmA sound (no more available in ArmA 2), the problem is I don't know what I have to put in the button class variable for the sound, I tried to put the path of the file and it doesn't work, I tried to define the sound in Description.ext and put the class name and it doesn't work.
Maybe I'm using a wrong path for the sound file, I tried to put it in missionFolder/Sound (where missionFolder is the folder of my mission). Do you know if that's correct?