Lately I've been annoyed with Hulu as the commercials are significantly louder than the actual show. This has caused me to wonder if there existed any application either generic or specific to Hulu or Firefox to reduce the difference in sound volume between the show and commercials.
The only way I know of:
Use the Virtual Audio Cable to loop back the sound into a sound processing server, like VSTHost or Console. Load a VST Plugin into VSTHost that implements a compressor/limiter. Set the output of VSTHost to your real soundcard.
Some of it is described here.
The compressor will increase the sound level of the quiet parts of hulu, but not the louder ones. Adjust the settings so that the shows and commercials are about even.
I know it sounds complicated, but there are many more things you can do with the VST host, depending on your CPU load.
If you are on OSX you can use Soundflower (Audio-Driver), this works for some applications.
Experiment with the settings.
That's it. Now you have compressed your sound in realtime.
Also you can apply any sound-effects as you like, e.g., a bit of reverb to make it more spacey, or apply some noise-filter (equalizer "remove s" (hiss)).
IMO the redirection to garageband has the best userinterface and no "hacking" involved. Minimize your garageband and enjoy.