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!

Here is a hard core way (read old school way) to do it. Use xev to tell you what the codes for the keys you want to use are. Then use something like (in $HOME/.xsession)

xmodmap -e 'keycode 171 = XF86AudioNext'
xmodmap -e 'keycode 173 = XF86AudioPrev'

to get the keys mapped to actions. Then you can modify your $HOME/.fvwm2rc to have

Key XF86AudioPrev               A       C       Exec exec nyxmms2 prev
Key XF86AudioNext               A       C       Exec exec nyxmms2 next

Restart fvwm2 and you are done.