How to Normalize Sound Volume (Yosemite)?

I am looking to equalize sound when watching MP4 movies on Quicktime.

I assume you are referring to "frequency" equalization here. If so and you insist on using the QT X v10.4 player, then use an external hardware speaker/amplifier with built-in frequency equalization controls. If willing to use an alternative software media player, then use one like VLC with its built-in "Audio Effects" (Equalizer, Compressor, Spatializer & Filter) control window.

How do you normalize/"frequency" equalization audio with VLC?

With your video loaded into the VLC player window, click on the "Effects" icon in the lower right-hand corner of the player window. when the "Effects" controller window opens, select the specific controls you want to adjust (Equalizer, Compressor, Spatializer, or Filter), make your adjustments, and repeat using different control if/as needed. Like QT 7 Audio controls, I believe the VLC controls are playback only.

If you want to adjust frequency equalization, add reverb and/or ambience, or apply compression effects as part of an audio editing and export workflow, then use an app like GarageBand and turn on the "Smart Controls" in the "View" menu ("B" keyboard shortcut) and make adjustments as desired. "Volume" normalization is also available in GarageBand as an export preference in the "Preference" menu if desired.

Since your topic title and actual submitted paragraph were somewhat different as to what you really wanted to do here, I have provided multiple answers this time.