I have been using Fedora 8 without any problem on the sound system for a month more or less.
Some days ago I changed somethings on KDE and since then, I'm getting the same results you are describing.
I test successfully my sound card using KDE-->system-->soundcard detection but it fails with KDE Control Center. Moreover, it fails as soon as I connect my user.
Well, I'm not able to explain any reason but (yum)erasing some packages, everything work again.
These are the packages I have erased:
ene 08 22:20:07 Erased: kdemultimedia-extras
ene 08 22:20:27 Erased: kdemultimedia
ene 08 22:20:36 Erased: kdemultimedia-extras-libs
ene 08 22:20:37 Erased: kdemultimedia-libs
I hope it will help to avoid the problem ... meanwhile