nope, you can't....
(not to mention it would be annoying to your contacts I think)
You can however make commands (aka shortcuts) to play a sound. For this use the
QuickText feature and the
/sound command.
See:
Official Help pages (general) :
http://old.msgplus.net/help.php
Official Help pages (commands & tags):
http://old.msgplus.net/help_cmdtag.php
Official Tutorial on commands & tags:
http://old.msgplus.net/help_tutorials.php?tut=commands-tags
Official Tutorial on QuickTexts:
http://old.msgplus.net/help_tutorials.php?tut=quick-text
QuickText Alias: /hi
QuickText Text:
/sound #
xxxxxxxxxxxx
#hi
Note: although the help pages are mainly for Messenger Plus! 3, most stuff still applies to Messenger Plus! Live.