First, I can assure you that linux is effin not driver friendly especially in Enterprise versions such as RHEL and CentOS. So let's get things started.
1. Make sure you have the SBx00 Azalia (Intel HDA) by typing at the terminal -> 'lspci | grep Audio' (case sensitive w/o the quote).
Having this audio device esp. in laptops, have high possibility that your experiencing no audio on the speaker, mic is not working or headphone jack doesn't work either. Editing the /etc/modprobe.conf and adding to the option line "model='whatever model alsa said on its website doesn't work either'".
If you manage to have a workaround on this without doing the next step. Please let me know.
2.If you tried mixing up the settings in 'alsamixer' and still no luck, go fire up the terminal and 'cd' to '/etc/yum.repos.d'.
3. Type 'touch ATrpms.repo'. Then 'gedit ATrpms.repo' for easier editing.
4. Copy and paste this to the file and save.
name=EL $releasever - $basearch - ATrpms
5. Copy paste to terminal:
rpm --import http://ATrpms.net/RPM-GPG-KEY.atrpms
6. 'yum install alsa-driver'
7. This will install 2 additional packages, the kml and the kernel update. After install, reboot.
8. You may also want to include the 'kernel-devel' and 'kernel-headers' just incase you will rebuild any device conflicts on the kernel update. (e.g. my fglrx driver for ATI)
9. After reboot, you can notice now that there's a headphone volume and addtl mic volume.