It may be that you have to open a mixer and turn the volume up.A lot of times it's turned down by default.

If that doesnt work,do this:
Open an xterm and type:
su -c "sndconfig" (with the quotes)
and press enter.It'll ask for your password.Type that in and press enter and the sndconfig utility should appear and walk you through configuration.
Dont worry if you dont see any characters appear while entering your password-that's normal.
HTH
lynch