It is quite possible that the earth connection on the tip ring and sleeve (TRS) plug has become open circuit. Both headphones are now in series and reproducing the difference between Left and Right in mono. And most of the time the vocals are cancelling out. You are only hearing the differences between the channels. You should not be hearing (or low level) other common things like bass, kick and snare etc..
Try reterminating the TRS plug. Cut off a few inches of cable to ensure you are terminating the TRS plug onto cable that is not faulty.
The earth part of the headphone socket could also be dirty and preventing the earth connection being made. Try cleaning the inside of the socket.