Find Lulu's Party

You focus on your goals again, and find the front door, which you open. You find yourself in a small entrance hall, with a staircase going downwards. This apartment must be the whole of the top floor. As you listen close, you hear music coming from a floor lower down. You follow the sound down the stairs and reach another floor, this time there are three doors. 31A, B, and C.

You try and listen from which door the music is coming from, but it's impossible to tell. You guess you'll have to try each of the doors until you find her. Which one do you try first?