The max volume on my iMac is way too low. I can barely hear videos in some online videos for example.
I know the volume can go louder though — depending on the audio source the iMac can be very loud.
So how can I increase the sound volume?
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.