[..]But: in alsamixer i can mute/unmute SPDIF but i cannot move the SPDIF sliders.
Not being able to move the slider is correct. The computer can't change the volume of the digital outputs easily.
(again something which should work ootb and does not....)
That comment was not really helpful to get you further, was it?! I understand that people are not happy, when stuff doesn't work for them, which should. But still, that comment doesn't help.
Did you check the wiki, as to what other things may help you? Are you able to output to S/P-DIF from running VLC or other audio output software?