1. To sing at a lower vocal register. You should really sing down to bring more power and volume to your voice.
2. To sing a note, song, or portion of a song at a lower register or octave. In this usage, a noun or pronoun can be used between "sing" and "down." Try singing the note down a half step. I think it will sound more vibrant. She took the pop song and sang down the whole thing in a dark, broody new style.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.