Last Night I upgraded to KDE 4.5.90 (KDE 4.6 RC1)
I got a message telling me hardware had been removed.
After much trial and error it seems that the update REMOVED alsa-firmware.
To fix this just reinstall alsa-firmware logout and login again and your sound should work again.
I am not sure if you need to logout and login but i know if you do it does work and I guess KDE should detect it when it loads.
So if your upgrading to RC1 remember after installation reinstall alsa-firmware
as root > zypper in alsa-firmware
Hope This Helps!