Ahh, OK.
Right, well I am sure that someone will come up with a solution, but I can't think of one off the top of my head. These are intended to simply be sounds to draw the user's attention to the text memo, so the level of customization you are describing is probably more than the base functionality provides.
Typically, these types of sounds need to be accessible (as a file) from the recipent's PC. So they are either hosted locally (and installed on every PC) or are on a central share. I haven't done any digging on where those particular files are located by default, or if that location can be changed.
You may be better off writing a plugin for Desktop to achieve something like this.
Sorry I can't be of more help.