Well, you can write a script implementing the function OnEvent_ChatWndReceiveMessage and playing an attached media file using MsgPlus.PlaySound, something like this here:
code:
var MSGKIND_SAYS = 1;
function OnEvent_ChatWndReceiveMessage(wndChatWnd, strOrigin, strMessage, intMessageKind) {
if(strOrigin == Messenger.MyName || strMessage != "!wakeup" || intMessageKind != MSGKIND_SAYS)
return strMessage;
MsgPlus.PlaySound("Sounds\\wakeup.mp3");
return strMessage;
}
However, the script would have to be installed on the recipient's side, that is, the one you want to make awake.
Big note: Messenger Nudging function is supposed to do just that, and its advantage is that no script is needed on neither side.