Some sounds already ingame have been configured at the main configuration file (\BIN) and are available at the trigger's effect section. Each sound has a class name associated, which means that you can call them with the 'say' command or the 'playSound'*.
example:
playSound "AKFire"
Note: "AK fire", "M16 fire" and "Explosion" available.
However, if you want to use other sounds then the ones specified at the config.bin, Knut Erik has the solution - yet try to config the "Description.ext" using a relative path (\Sounds\Environmental\Thunder.wss), instead of placing them at the mission folder... If it works, that is
.