Fedora Core 3 probably uses alsa... you could try running "alsamixergui", "gnome-alsamixer" or "alsamixer" from the console. Try to change all settings that might be relevant. If "alsamixer" refused to start, then you don't probably have alsa installed. You could also try starting the mixer program as root, to see if it is a privilege problem. If it is, check which user groups (e.g. "sound", "mixer", "disk") are available and add your user to the appropriate one. Then log out and log in again and try adjusting volume again.
Also check that you don't have any Sound daemon (artsd or esd) running - they might cause troubles.