I have encoded hundreds of video files with Premiere 6.5's Windows Media Encoder feature over the last few years but this past week noticed a prominent hissing (white noise) in the audio of the encoded video. It was only present in the audio after it was encoded with Windows Media Encoder and not with any other exporting function. After checking other possible sources for the cause of the problem and before uninstalling and reinstalling Premiere, I simply performed a complete reset of Premiere by pressing the Ctrl & shift buttons simultaneously while opening Premiere. If you do this, you will lose all preset settings for how Premiere's workspace is set as well as its presets for capturing and new project settings. However, all pre-existing projects' settings will be retained. This reset fixed the issue and I was really happy! This complete reset will also fix a known issue with the Export Movie function.

Well, I thought I had it fixed but it happened again. At least I know how to fix it until it happens again though. I guess I'm going to have to do a re-install of Premiere to hopefully resest whatever it is that's causing it.