Boost output volume past the maximum on OS X
Solution 1:
Mac App Store - Boom ($8.99):
You could modify the instructions at osx - OS X: Filter system audio-out to remove high-pitched noise? - Super User. That's too complicated to be used as a permanent solution though.
iTunes has an option to normalize the playback volume: Preferences → Playback → Sound Check. As does VLC: Preferences → Audio → Volume Normalizer.