There have been cases where the volume is muted by default, and this isn't exposed in the KDE mixer. To fix this, run padevchooser (you may have to install it), click the tray icon, and choose "Volume Control". Check that everything is not muted, including after you start playing sound through an application (the application should show up in the "Playback" tab).
If the sound still isn't working, or the pulseaudio tools aren't working, try adding your user to the "pulse" and "pulse-access" groups (Yast > User and Group Management > [select user] > Edit > Details > Additional Groups). You'll have to log off and back in for this to take effect.
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