I want to clarify/correct two things:
(1) The most important issue is the impossible access to /dev/mem, even being root. This character device is registered in /dev/ with major=0 and minor=0. user is root, group is kmem and rights are rw-r-----. Apparently this prevents full use of the GPIO as we have to map memory regions, like in the code snippet given at the end of this email (taken from http://elinux.org/RPi_Low-level_peripherals#C
(2) Concerning audio output, everything is fine now, except that I have no clue how to control the volume level and that it is impossible to do it from within mplayer (no error output, but controls are ineffective).