The vocals are out of phase. Panned wide for an effect but.... , it sounds great in stereo but if the sound source isn't playing it back like your source is, or is being summed mono, its going to cancel the vocals like you are hearing now. Always check your vocals ( all stereo tracks) in mono. If it sounds right in mono, it will definitely sound right in stereo. The trick is learning how to get that wide sound you want without phase cancellation.
If you can't do this in you DAW, I highly recommend a monitor control system that allows you to audition music in mono.