I just installed Windows 7 Pro 64 on my Dell, after having increased mem to 4Gb. I do have sound for system events and when playing MP3 files from Windows Media Player, but when trying to watch YouTube vids or listen to Pandora in Firefox or IE9 I get no sound.
I tried reinstalling the latest Flash Player directly from Adobe site, but that didn't remedy the problem.
I checked sound properties in the Control Panel Sounds applet, but everything appears to be in order there. After all, I do have sound for system events such as startup, shutdown, beeps, etc. and when playing MP3s in WMP. Just not online.