How can I adjust by how much the sound volume changes every time I press the volume change keys?

"You want to change the volume step interval, this is fairly easy.

Open up the terminal and run the command gconf-editor, it will bring up this window:

In the left pane, expand apps and click on gnome_settings_daemon:

double click the volume_step key in the right pane, enter the new step value (this is the value you want the volume to change by when you press volume up or volume down on your keyboard), and press OK:

test your new configuration and exit gconf-editor if all is working!"

"I assume you're using a laptop or one of those enhanced keyboards..? If so, careful because

ubuntu 9.10 has pulse audio; aumix is alsa-related and might take you down the wrong path
what you need is probably something like xmodmap (if it can capture your key), or if you're on a laptop there should be a kernel module/applet able to capture the specific key press. On some (my old ASUS) I had to go through ACPI to access the volume key."