Nasopharyngeal carcinoma causes deafness by

Due to the obstruction of the eustachian tube, there is a conductive hearing loss with serious or suppurative otitis media. Tinnitus and dizziness may occur. Presence of unilateral serous otitis media in an adult should raise suspicion of nasopharyngeal growth. Rarely, the tumour grows up the tube into the middle ear. (Ref: Diseases of Ear, Nose and Throat, P.L Dhingra, 7th edition, page 284)