its simple, go to preferences, once there, click the personalize [dunno name in english
] tab and go to "event notifier"
Here you can receive notifications of the different actions of your contacts with an emergent toast, sounds or both
you must chick add and select one or all contacts and put an action. Then you can chose to toast or a sound to be made when this contact has made the action [or both].
edit: oh, -!Felu!- got it first