Shoutbox

afaik plus dont have this feature yet.. u can assign signin notification sounds for different contact but not diferent offline notification sounds for differenct contacts...
there is only one (universal )  sound u can assing for all of ur contacts.. u can change that universal ofline notification sound from plus preference panel > notification > popup option > sound file ( browse to the sound u want to play for offline notification ) > open > apply > ok ....

if u want further help then ask for more...