The only reason why you have to put it there is because you can't have two functions of the same name in the script. OnEvent_ChatWndReceiveMessage() is simply a function that is called when you receive a message. In your script, if you receive "Razzia" the sounds will start playing. If you receive "QUIT" the sounds will stop playing. That's basically how it works, you don't need to copy it after each sound.
/ 
