Yep, I've found since moving from RH9 to FC4 I've had similar issues. Typically I'd get an error on boot up stating "invalid argument - sound being set to null".
I was able to get my sound working by:
-> I tried all the "audio devices" under control center>sound system>hardware. And found that "autodect" worked the best, [your setting may be different?].
-> I unchecked "Auto-Suspend" at the bottom of "General" and all my system notice sounds came up.
-> I ran kmixer" and made sure that "external amp" was enabled. One issue I had was after enabling the volume control [under "kmix configure>settings"], it disappear when I closed the program.
I found instead of closing the program, I right-clicked on the speaker icon and selected "hide mixer" and it's worked ever since. I wasn't able to find this info in Help so I just experimented.
-> I verified the settings mentioned above in "alsamixer" [run in terninal]. Tab to "all", arrow to output, and select "m" to mute or unmute.
-> Checked my "Blacklist" file [under /etc/hotplug/], and removed the driver for my soundcard. Use this with caution, there may be a binding issue, but have to say my sound works now.
-> Lastly I noticed in Linux, that the actual sound volume is between 85-100%, the sliders offer very little resolution. Many have found just turning them all the way up gave them sound...