If you can recall, last year, Taeyeon made her musical debut in ‘Midnight Sun’ and took the role of Kaoru, a girl who had xeroderma pigmentosum (a disease that didn’t allow her to be exposed to the sunlight so she couldn’t go out in the daylight since she couldn’t be exposed to the sunlight), yet she lived happily day by day, crushing on the main actor, Koji, and going out at night to sing.
I believe she participated in 5 songs in the whole musical and studio versions of 3 of them have been released–‘It’s Happy Line’, ‘Goodbye Days’, and ‘Skyline’. However, these are re-mastered fan-made versions.