There is a variety of things that could be wrong here, and I'm no expert with the custom sounds feature, but I can touch upon it.
Are the sounds you are attempting to send the same? Is it the same file? There may be a problem with that file, which Plus! cannot convert (I believe the LAME MP3 encoder is used).
It could be your contacts have the option not to receive sounds, or download them without clicking a button. Tell them they have to press play, or turn the option on ;
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