Shoutbox

Actually, that's part of the reaons why the search box was added in Messenger Plus! Live. If you want to "group" some of your sounds you can always rename them by adding something like "(My Category)" at the end of their name and enter "My Category" in the search box.

Custom categories could be pretty hard to develop and may not be very convinient, still, it's an idea that could be developed (it would only be for local use, the server needs fixed categories so it would be more "custom groups" than "custom categories").