As far as I know, the sound is global for the entire system. There is no way to really turn down for one Safari window only. However, you can turn down the alert sounds separately from the rest of the system sounds. You could also possibly turn down the volume if what you're accessing requires a plugin like QuickTime or Flash....then you would be able to play around individually with the sounds for each media file being accessed by said plugins through their player's volume control. But as far as I know, that's about it.