Lyric discussion by saintaequitas 

Cover art for The Mixed Tape lyrics by Jack's Mannequin

I think he says bitch, not that it matters. In iTunes, the song is labeled as explicit. So unless the English language adapted another "explicit word" that I'm not aware of, he says bitch.

There is obviously a "clean" version of the song, and that is why some (most?) people hear "mix", I hear bitch.