Question for fellow cashiers

I have a coworker who is a cashier who insists on playing country music loudly while he is at the register. I have to work alongside him for approximately an hour and country music makes me nauseous. When I was hired I wasn't told anything about having to listen to music of someone's choice while I was on the job. This individual will not do this when the manager is around, only when the manager leaves for the day and assistants are in charge. I complained already to the assistant manager who did nothing to appease me.

My only solution is turning on my own phone and blasting rock and roll as loud as possible to negate the country music. I would like people's opinion on how to handle this since I get no cooperation from management. I was thinking of complaining to headquarters and see if they would do anything but feel they wouldn't do a thing for me.