vocalism vs voice over

vocalism and voice over both are nouns.

Word	Noun	Adjective	Verb	Adverb
vocalism	Yes	No	No	No
voice over	Yes	No	No	No

As nouns, voice over is a hyponym of vocalism; that is, voice over is a word with a more specific, narrower meaning than vocalism:

vocalism: the sound made by the vibration of vocal folds modified by the resonance of the vocal tract
voice over: the voice on an unseen commentator in a film of television program

Other hyponyms of vocalism include singing voice, sprechgesang, sprechstimme, voice.

vocalism (noun)	voice over (noun)
the sound made by the vibration of vocal folds modified by the resonance of the vocal tract	the voice on an unseen commentator in a film of television program
the system of vowels used in a particular language