I use a similar DELL at work and had sound issues. Check DELL support for your model and check to see if there are updates for your motherboard and audio drivers. When they gave me my computer fresh and shiny and new at work I had to do that before the sound worked at all.
I've noticed there are a bunch of tech-savvy people on this board, so I'll seek help here. If you read the subject you see my problem. More detail: In many cases (but not all) I can get the sound for the ad, but when the video starts, there is no sound.
Constantly updated XP
IE 6 patched and updated
Dell Pentium 4 2.6GHz 512MB
Default sound device:SoundMAXDigital Audio
Legacy Audio Drivers
I hope I've provided the right clues(probably not). Thanks in advance for your time and advice.