I am working on a projekt and one part of it is an control panel for my pc 5.1 audio output.
My question is: can I use an Arduino to change the volume off single speaker pairs (front back center base ) ???
Yes. You can control several (or 6) digital potentiometers.
If there are keyboard commands to change the volume then an Arduino Micro can do it by USB. The extended "media keyboard" has lots of key commands you probably never thought of.
Is this 5.1 output analog (6 RCA plugs) or digital (optical or coax)?
analog
So Doug's solution will work, so long as you can find digital pots which allow their potentiometer terminals to go below zero volts, with the audio signal.
If you wanted to go for "cool effect" (if your "control panel" is supposed to be a physical device) - you could also use motorized potentiometers (log taper - not linear taper). Though it wouldn't be cheap, it would look interesting (and you could still have a manual method to control the pots - though you would need some extra hardware and software to sense the current rotation position of the pots).