I am having the same problem. I'm running a PC with Windows XP, and about two months ago I upgraded from 1.5 GB of RAM to 3 GB (actually 4 1GB sticks, but the system only recognizes 3 GB). During some games, for example The Witcher and Age of Conan the sound cuts out during CINEMATIC cutscenes, playing fine for a few seconds, then cuts out for about 10-15 seconds, then comes back and continues to cut in and out. There are no problems with the sound elsewhere in the game, such as in the menus, in the actual gameplay and in in-game, non-cinematic cutscenes. I have tested various other kinds of video files that I have in windows media player and VLC player, and they all seem to play fine. Cinematic cutscenes seem to be the only source of trouble.
All my drivers appear to be up to date, particularly the video and sound card drivers, but I did recently reinstall windows in addition to adding the new RAM and a new hard drive. Before these changes the cinematics in these games ran fine with no errors, but I cannot for the life of me figure out what is causing this change, either through testing or research online.
Please, if someone could at least give me some general idea of what might be causing this problem it would be greatly appreciated.