Should I put soundtrack lyrics under no follow tag to avoid getting penalized for the duplicate content, because the lyrics of every song would be the same therefore identical on other websites ? Or should I not worry about this because it doesn't matter ?
nofollow tag is for links, not content. You can try noindex... but it won't change much at this situation.
i've got copied lyrics from metrolyrics on one of my websites and i haven't been penalized by Google. you could also wrap lyrics copied in QUOTE tags.
if you are not looking for traffic from those lyrics when indexed (might or might not come) then your best bet is to disallow indexing of those pages, at least you will keep the pr juice to the pages that will actually rank.