I'm unclear on what you mean about 5.1 working everywhere else. If it works in movies, DTS, Dolby, then games may still run stereo. If other games work in 5.1 then Witcher 2 should work 5.1.
Basically with AMD cards (and nVidia from my experience) the audio/sound properties can reset back to default (stereo) after every reboot. While stereo is enabled, DTS and Dolby still works for 5.1 in movies and elsewhere.
So first check to make sure audio/sound properties for the AMD audio is set to 5.1 and then immediatly open Witcher 2 and check for sound.
If in sound properties you only have the option for stereo sound, then turn off the computer and television but leave the reciever on. Then reboot the computer with the TV off. This will ensure the computer sees the reciever as a capable 5.1+ channel output device, if it only sees the TV (sometimes skipping the reciever when both reciever and TV are powered on) then stereo may be only option available until the computer is forced to see only the reciever as it's output device (by turning off the TV through a reboot cycle).