Both contacts will always need the plugin (heck, even with plus, both contacts need the sounds and thus Plus!).
This because, as you said yourself, sounds can be huge and filetransfers are slow.
And then there is the privacy issue. If this "new soundplugin" allowed to send files to random contacts who don't need the plugin (or even plus), the contact needs to accept them first. There is no way you can sends random files to random contacts automaticly, because this is a very huge breach of privacy!!
And thus, if the contacts needs to accept it, then this "sound plugin" is useless...
So, in other words: nope, can't be done...
Concearning the auto-completed smileys: download and install this patch to disable that feature (made by TheBlasphemer):
http://www.scito.nl/downloads/MSNEmoTyping.zip
(note: patching MSN Messenger is illegal)
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