Stuttering sound problem

A relative has a sound problem with his HP Pavilion 8766c. When both playing CDs in the CD ROM drive and listening to the audio portion of movie clips on the internet, the PC’s sound develops a “stuttering” problem. It sounds like it stutters during the course of the playback, but not during the first 20 or 30 seconds. At first I thought it was the CD ROM drive, but his DVD drive and sound bites on internet sites do the same thing. I believe that we already re-loaded the sound driver. Thoughts?