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One non-optimal possibility is download the video and then play it in VLC which has the ability to increase the volume up to 800% (using the mousewheel on the volume slider).

If you are using Windows(*) you can mute an application. In your case, you could mute the browser you are using to watch the videos. I don't know if this feature is present in other OSs and how it works. (*): I'm using Vista

You could use the fine Mac program Audio Hijack Pro to amplify your entire system's volume while you watching Netflix, as described here.

Presumably flash is accessing you audio at the driver level so the only change you could make would be there i.e. using a driver that lets you route the audio through processing (It might be worth looking into this http://jackaudio.org/ I'm not sure this will do do the job). Perhaps HTML5s video tag will open up the ability to do this sort of thing with a ...