Using the windows version of KDE, Konversation notifications do not make sounds at all.
confirmed: there are not even default sounds available on windows.
Ok, I just talked to a konversation developer and he told me that there are no default notification sounds available in konversation. If you still want to have sounds, you can select them in the 'Configure Notifications' dialog. In that dialog you should be able to add new sounds. If these sounds do not work (using the test button) your phonon setup is probably broken (e.g. a missing vlc package) but that would be another bug.