The 5 winners from each field of 16 will square off in some fashion. I vote for a one-round single-transferrable-vote style round, but there's plenty of opportunities:
If you aren't doing 1-on-1 matches at the end after doing them throughout the tournament, it will completely ruin the integrity of the bracket. Just have a vote-off at the round of 10-that-should-be-16 for some second chance songs.