Disable fadeout on mute

Since Mavericks, when I mute the sound, it fades out instead of mutes instantly as it did previously. I'd like to change the behavior back to what it was. When I press mute, it's because I need to pay attention to something going on around me at that instant, and fading out doesn't allow me to do that. Is there a defaults write that I haven't been able to find yet?

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I tried to fill out the OS, but the form was stuck on iOS.