The Flash/Supergirl crossover musical episode "The Duet" will air on March 21, 2017 - the 17th episode. Villain is the Music Meister, who also appeared in the animated series Batman: The Brave and the Bold.
Confirmed cast members who will sing in the episode are all the ones who have musical backgrounds: Grant Gustin, Melissa Benoist, Jesse L. Martin, Victor Garber, Carlos Valdes, Jeremy Jordan, and John Barrowman. Gustin and Benoist were once part of the TV series Glee and have performed on stage musicals (notably The Sound of Music and West Side Story), while Martin, Garber, Valdes, Jordan, and Barrowman have extensive stage experience on Broadway and London musicals.