The sound from the iMac speakers when using 64 bit Windows 7 is not as high quality as Mac OS X. Here are a few things you can do to fix the sound, or rather few settings that change the sound a little.
First make sure your Cirrus Logic drivers are up to date, you can download them here. Go to Device Manager (in Control Panel, as Administrator), select Sound, video and game controllers, select the Cirrus Logic driver and right-click and select Update driver software and then choose Browse my computer for driver software, pointing it to where you un-zipped the driver you downloaded. This step should make sure that Bootcamp did indeed install the latest drivers.
The second step is to change the the way Windows 7 thinks of your speaker organization. Select Sound (in Control Panel), in the Playback tab, select Speakers and click the Configure button on the bottom left. For me Surround works best, by default Quadraphonic was selected and changing this to Surround seems to work better.
Maybe I'm imagining things, but this does sound better to me, if you figure out something else do leave a comment.
