I'm not sure if there's a better way to do what you want, but there's an easy workaround
Install "amixer" (sudo apt-get install amixer), and then you can use the custom shortcuts in GNOME: run "gconf-editor" then go to "/apps/metacity/global_keybindings" and add the shortcut you want to use as the key for run_command_1, then go to "/apps/metacity/keybinding_commands" and enter "amixer set PCM 10+" into the command_1 key value. Then, hopefully, whenever you press the shortcut it should raise the PCM volume level by 10 decibels. Similarly, you could set command_2 as "amixer set PCM 10-" to lower the volume. Hope this helps