I am using Win7 x64.
I have Polish retail disc version of the Witcher EE installed and I can enable or disable EAX in game menu.

However, when i installed and run GoG version, I noticed that the EAX checkbox is greyed out. Any ideas on how to fix it? Creative Alchemy?

I'm not even sure this is accurate to be honest. I use a dolby digital decoder mixamp and a set of headphones (Astro A40's) and I'm fairly sure I get the surround sound effects as sometimes voices etc still come from people stood behind me. It could be because I have AC3 Filter forcing all outgoing signals to be 5.1 though as I have one of these stupid realtek chips that, despite being designed for Windows 7, seems unable to output in anything more than stereo.

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I have alchemy installed with a Creative Audigy board on a Vista-64 machine. Interestingly enough Alchemy detected "The Witcher" automatically; the only game I ever recall being detected. Usually I have to set it myself.

This is something I have noticed is pretty standard for the current generation of console games. With a pair of stereo headphones sound that come from behind the camera sound like they are coming from behind you. Its pretty sweet. I cant really comment on PC games. I just installed a new Radeon HD 6950 a couple of days ago. Before that I wasn't able to play any recent-ish games, so im not sure how common it is.

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https://steamcommunity.com/app/20900/discussions/0/540743212939537023/ - here is easy manual. Just change "Steam" folder to "GOG folder" :)

Good Luck !

we have to Configure the Destination-Folder from the Game where the "witcher.exe" File is located.
"E:\Games\GOG Games\The Witcher Enhanced Edition\System"
E is my Drive/Storage , Games ist my personally Folder where i installed my gog-games, in this Fall the exe-file from the Games is located in System Underfolder of the Game-Folder.

When you have a real SoundBlaster that can EAX it is a bad Idea to use the "Alchemy Universal" thats only the best choice if you don't have a Sounddevice that support EAX.

Another one as Example is to use "dgvoodoo2" with Duke Nukem Forever for getting MSAA in the game an Wrap it to a newer DX API.
This Game has too an Underfolder where the exe-file located is.
The exe-file uses Librarys that are stored in his own Folder, another way it uses Systemlibrarys, that's the Trick, we have to configure in Alchemy the right Destination an then it puts the dsound.dll (library) File and the dsound.ini (config file) file in it.