Hi,

I did a preliminary googling, but found no result to this problem. After playing the game for about 5 minutes, the sound starts to skip, i.e. going silent for a couple of seconds, then resumes. As I spend more time in-game it appears to become more frequent.

Take a look at this thread maybe it help you out. You need to download the binkw32.dll file and replace it, as explained in the thread.
http://forums.steampowered.com/forums/showthread.php?p=35099094

Afaik, this fix only relates to stuttering cutscenes (which are played by the bink library) and not to the normal in-game sound. Still, if nothing else works, it doesn't hurt to try. The stuttering problem could occur for many reasons.
Here are some shots in the dark:

The Witcher showed stuttering on certain multi-core systems.
You could try changing core affinity to only one CPU (or if that works to two CPUs max).
On Windows 7:
- open the task manager while the game is running
- on the Processes tab look for The Witcher 's executable. Right-click it and choose "Set Affinity...".
- Untick all CPUs but one
- Apply changes and go back to the game
I'd try it before the sound glitch appears.
Unfortunately core affinity isn't saved, so this has to be done every time you run the game (or you write a custom start script/batch file).

It's always worth a try to run the game in Compatibility Mode for Windows XP (only works for Vista and Windows 7).
- Right-click the game's start icon => Preferences

If you're using fast hardware, try enabling V-Sync in your graphic card's control panel.

If you're actually using Windows XP, I'd try lowering the level of sound hardware acceleration, maybe there's some sort of incompatibility.
- Press Windows-Key + 'R' to open the Run dialog
- type dxdiag and press Enter
- on Windows XP the Sound tab offers an option to lower sound hardware acceleration (lower it one level for a start)