For the longest time I had the same issue. Then one day as I was going through some of the posts here on sound I figure out what I needed to do. Assuming you get sound elsewhere and using KDE (not sure of GNOME but I suspect there is something similar), give this a try (if you haven't already):
Press the
K start menu and go to --->
Settings --->
Control Center. In Control Center pick
Sound and Multimedia. Then select
Sound System. One of the tabs should be
Sound I/O. Select that and for the drop down for the
Sound I/O Method select your sound Driver rather than Autodetect. (in my case it was Advanced Linux Sound Architecture). Save it and try again.
Hope that helps.
