When I switched over to Xfce 4 from Gnome 2, I noticed that my sound stopped working. I spent a while Googling and reading other users fixes for the issue. A lot of the solutions I found, did not work for me. I am going to list the 3 steps that worked for me when figuring out the audio problem.
Pulse audio and Xfce seemed to just not get a long. So I removed it. Multiple websites and threads I read stated to do this.
# yum -y remove pulseaudio
If your sound is working now as a root user, but not as a normal user, try adding yourself to the audio group. (I don't like this fix, but till I have a better solution this is it).
Check to see if your already part of the audio group.
# id your_username
If your not part of the group, add yourself
# chmod -a -G audio your_username
After making the changes above, log off and log back on.
If it's still not working, try playing a audio file from the command line. If your getting a permission error, you probably don't have access to the sound devices in /dev/snd.
Play a sound from the command line
$ aplay /usr/share/sounds/alsa/Noise.wav
Check the permissions for the sound devices in /dev/snd
$ ls -la /dev/snd
Does the audio group have permission? If not add the group.
# chmod -R root:audio /dev/snd