so you already have kde configured to use gstreamer.
The problem could then be on the playback device configured to be used to play log in sounds.
At the dialog window that allow you to change phonon backends, click the "device preference" tab and then click "audio playback" on the lower left and you will see a list of audio devices on your lower right.
click the first entry on the right and then click the "play" button. Do you hear sound? If no, try another one on the list and move up to the top the one that can play sound
Do the same for "notification","audio" and "video" .
You are not getting sound probably because of them is configured to use a non working audio playback device