I noticed lately when I play Yahoo! HD movie trailers in my browser, there’s been no sound and it always turns out to be that the volume is set to the lowest. I thought I might’ve accidentally lowered the volume, but if I increase the volume and reload that trailer, the volume goes all the way back down again. I then went ahead and tried an Apple HD movie trailer and they don’t have this problem. If you’re downloading the trailer, it doesn’t have this problem as your application most likely retains its settings, but for some reason if you use the Quicktime player in your browser, their trailer is able to control your volume setting.
I’ve verified this is the case on Firefox and when I tried it on IE7, the same thing occurred. So if you’re trying to watch a Yahoo! HD movie trailer in you browser and you get no sound, just go check the volume control as that’s very likely to have been lowered to no sound.