I have KDE-3.5.10 on 6 PCs, each with two users accounts and I do not experience this problem. However I have NEVER logged in as user root. In your first post you noted you logged in as user root. Have you done that log in as root many times? I am wondering now if that log in as user root messed up your configuration.
Am I correct now that sound works for all users (except for firefox which causes one user problems) ?
I confess to have logged in as root many times 
I can get sound to work for all users and programs except browsers.
On user d I have no problem getting sound (and video) with Firefox or Konqueror, but Konqueror did complain of a script causing KHTML to freeze however the sound came anyway!
On user j I have video but no sound on both Firefox and Konqueror (no complaints from K!)
I created another user g just to see how the defaults would work. For user g Firefox gave video but no sound and Konqueror gave neither video nor sound and complained of a script causing KHTML to freeze.
Regards Schrod
I can’t explain these differences.
You could backup the directory /home/user-j/.Mozilla and then try a copy of the directory /home/user-d/.Mozilla to /home/user-j and may be also restore user-j’s original bookmark file. See if that helps user-j. You may need to adjust permissions for user-j to use user-d’s .Mozilla.
Again, you MUST ensure user-d has shut down firefox properly, before logging out.
I recommend you NEVER log in as user root. My apologies, but I make it a practise NOT to help users who log in as user root, as that can introduce unexpected effects and problems that I can not help with, and it can end up being a time waster. Sometimes a re-install of the entire OS is the only quck way to address problems introduced by inappropriate log in as user root (which is why I never log in to the desktop GUI as root).
I’m afraid that copying user d’s .mozilla directory to user-j did not change the symptoms. I will have to re-install when I have time 
Thanks for all your time and help.
Schrod.
I’d the same problem. I logged as root and ran Firefox, I got sound and now it works as user.
Regards.
Michel.