i had a similar problem and i just did the following. open a prompt and run 'aumix', turn all the volumes up, set it not to mute, and save settings. then run 'kmix', and turn all the volumes up. There may be some alternative mixer you need to use in GNOME, not sure.
Now it sounds like i am just telling you to turn up your volumes, but the actual fact of running these utils seems to create the necessary symlinks for my sound to work, so there you go. hope it is useful to you.