Thank you for advices but you now that the default setup of Ubuntu uses a combination of ALSA and PulseAudio to output the sound. I installed Open Sound System because it's much better and doesn't make so many problems. If you have it installed you don't need PulseAudio to mix many sound sources (for example two media players and flash). I solved my problems with no sound in The Witcher 2 I just have to disable vmix (OSSv4 mixer) in the ossxmix before playing the game.
