This is for those of you with sound cards or USB headphones (may also affect on-board optical audio).
If you are experiencing no sound in game but have no issues in windows then open up your audio software (xonar, creative, etc.) and change the setting for the number of speakers/channels to 5.1/6 channel or below. This issue appears to be affecting any system configured higher than 5.1/6 channel (ex 7.1/8 channel has no audio).
 
If you don't have audio software for your device the same thing can be done in windows (Note I'm using W7), goto Start>Control Panel>Sound
Then in the window that popped up goto the "Playback" tab and select your device then click the "configure" button. In the window that pops up seect somthing below 5.1 and your sound should now work in game. Remember the steps you took in order to reverse them later.  
I don't have a multi-channel device hooked up so I can't see the setting but I believe it's under the "advanced" tab in a drop-down that lists the audio channels. Select 6 channel or below and click apply.
 
TL;DR The Witcher 2 can not be played above 5.1 / 6 channel sound or the audio will completely drop out.
 
EDIT
If this works or doesn't work for you please post to let me know either way. I'll see if I can help diagnose your issue.
 
UPDATE 05/31
As you may be aware the game has been patched to 1.1 this patch has resolved most audio issues. You can now switch your settings back and see if they work, I'm now able to use 5.1 or 7.1
 
Those of you with Logitech G35 7.1 headsets there is still an issue with audio via USB. The devs have been made aware and it may be one of the issues addressed by the next patch 1.2. If you would like to draw more attention to the G35 issue you can visit http://en.thewitcher.com/community/ and post letting them know. Note the main boards are currently down, but they are monitoring the comments.
 
If you have sound stuttering issues after the patch in 5.1 or 7.1 it is an issue with the update. It can be fixed by uninstalling the game, note you CAN keep your save game, and reinstalling it and the patch. You should now have full uninterrupted audio once again.

Thanks for letting me know, I've corrected the main post.