Well... yes and no. :) As with most such performances from 60s and 70s television, the singers and musicians are just lip-synching to the studio recording, not doing a live performance. (It's painfully obvious if you watch Mendes' hands; notice often he's completely out of sync with the actual piano notes?) So, the powerful vocals are in part due to Lani Hall and Janis Hansen being double-tracked in the studio mix.

Which isn't to say they aren't still fine singers, especially considering that they had to learn all those lines phonetically since neither Hall nor Hansen spoke a word of Portuguese. :)