A member of a choir, or show choir, that is extremely obsessed with it. These people tend to sing non-stop, do the choir's vocal warm ups when they are bored, know every part (Soprano, Alto, Tenor, Bass, etc.) to a song, and prove that they know everyone's part by singing it all the time.
When a group of choir nerds is at a competition, or on a field trip of some sort, you can usually spot them trying to sing every song from every Disney movie or musical imaginable. They are seldom on pitch when doing so, and seem to have A.D.D. when singing. They get past a few bars of music, and then they get bored and look for another song.
Ryan: Hey na-ne-ding-dong, ya lang-yalang-a-lang
Mark: Boop sha do, ya-tatatatatata
Rachel: MARK! RYAN! You are SUCH choir nerds!