as for using the shoulder button as hotkey, depending on the player its possible. for Audacious its in plugin under global hotkey (or something like that). now one issue, audacious wont let you map modifier key (alt, shift, ctrl etc) directly as an hotkey since it expect you to press another key in combination.
One way to work around that, would be to use xmodmap and remap the shoulder button to some other key (could even map them to keyboard media key like next/previous track), now as to which code is the shoulders, im not sure anymore, been a while since i messed in here, but well, thats at least something to look into.