Recently I experienced serious problems with using KDE's audio system, which I eventually tracked to the program 'kmix.' The symptoms were extremely slow response on volume-changes and mutes. By slow, I mean taking a full minute or more to apply. Also, it was taking up 100% of the CPU sometimes for minutes at a time (when it was not responding). Thank goodness I have a quad-core.
I am not sure if the problem is the result of using KDE 4.6.0 or not, since it is very recent and it might have introduced some bugs.
Regardless, I eventually figured a solution to the problem: Quit kmix. You may need to actually kill the program. Next move (or delete or rename) the folder 'kmix' in the folder '~/.kde/share/apps/. Restart kmix, and it will recreate a default '~/.kde/share/apps/kmix/' folder with default settings.
This fixed my problem. Hopefully it will help with yours.