The website has not been updated for a long time, but some basic instructions about using configuration files have been there for ages: http://mplayer.garage.maemo.org/

You don't need to be root in order to edit mplayer configuration file.

Regarding the sound volume, make sure that you have set it to maximum in both device control panel applet (global system-wide volume settings) and in mplayer by pressing '+' button until it is maximized.