From main WLM screen Plus > Sound Library > bottom left: Import/Export ..Export to Soundpack
To make them mp3 you can simply copy them to another folder and then rename the extension mp3 (do not rename them back and put back in Plus sound folder).
You can find the sounds in .dat format in
C:\Documents and Settings\All Users\Application Data\Messenger Plus!\Custom Sounds
For converting all sounds in a soundpack to mp3 have a look at
Soundpack Extractor.
Edit: you're welcome
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