I haven't scripted anything for a while now, let alone visited the forum, but I think it should be possible to check the sound library for the existence of the sound before sending it.
Custom sounds can be found in:
C:\Documents and Settings\All Users\Application Data\Messenger Plus!\Custom Sounds\
and
C:\Documents and Settings\<Current User>\Application Data\Messenger Plus!\Custom Sounds\
At least, in WinXP. Vista will probably use a similar location, so I think something like
%AppData%\Messenger Plus!\Custom Sounds (and
%AllUsersProfile%\Application Data\Messenger Plus!\Custom Sounds) will be the best option.
The name of the sound as Plus! will display it can be found in Properties>Attributes (not sure about the names, I'm on a dutch machine), so you'll have to find a way to access this information.
Now I don't know if this method takes all possible scenarios into account (default sounds, sound packs, and the likes), but this is what first came on my mind.
Good Luck
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