If you use multiple X sessions under Fedora Core, like me, then you might have noticed that sound only works under the first session that you logged in from.
The problem is that the sound devices in /dev/snd are created during login. However the default owners and modes for these devices are set to the user:root, and 600 (at least in Fedora Core they are). That way when another person logs in, it doesn't have read/write permissions for the sound devices.
The solution is to change the default owner / mode that are set for these files during login. Open the file /etc/security/console.perms and look for the following line:
0600 0600 root
And change it to:
0660 0660 root.users
That way all members of the 'users' group have acces to the sound device. Of course, if you want to set other groups to have sound permissions, all you have to do is change 'users' at the end of the line to whatever group you want.