If you are using KDE then you are probably using aRTs. It is a sound-system that makes it possible for multiple programs to make sound at the same time.
Under the KDE Control Center go to Sound & Multimedia -> Sound System (not at my Linux box at the moment so I can't remember the names exactly). There should be a slider that adjusts the buffer-size. If you haven't arts, you can download it from www.arts-project.org
Also, remember to download (and activate) the arts-plugin for XMMS, so it directs the sound output to arts.