Google this.
Disable Flash Hardware Acceleration.
And post this in the XP Forum where more folk discuss XP.
Bob
I have Dell desktop XP operating system. My YouTube audio just disappeared but if I download it and play it using GOM player there is sound. I lost audio sometime before and just came back 2 days later without doing a thing. This time it's one week now and no miracles. What's the solution? (Sometimes some news clips have no audio too).
thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic