What version of ALSA driver are you using? Have you tried updating ALSA as the version within CentOS is quite old now (1.0.14rc3).
Elrepo has an updated ALSA driver package here:
Thanks for that useful information, but can you tell me exactly how to perform the update? I tried 'yum update *alsa*' as I have the Elrepo repository installed, but it came up with 'No Packages marked for Update'. Do I need to completely remove all the currently installed ALSA packages and then do 'yum install kmod-alsa? Any advice gratefully received, thanks.
Andy