Do any of your KDE sounds work? When you say you've 'tried everything' you need to actually lay out the exact steps you have gone through in order for us to have any idea how to help you
There is a general sound troubleshooting FAQ for KDE here:
http://docs.kde.org/development/en/kdebase/faq/sound.html
If your sound does work properly in other KDE applications, try renaming `kde-config --localprefix`/share/config/kopete.eventsrc to something else and then reconfiguring your kopete sounds. It has happened before that they've been somehow swallowed on updating Kopete versions while retaining old settings.
(Kopete sounds are not enabled by default and have to be enabled through Settings -> Configure Notifications on the Kopete buddy list window)
Hope that helps!