The present inventors report here, in the DFNB9 mouse model (OTOF knock-out mice), the first proof-of-principle that cochlear delivery of a fragmented cDNA via a dual-AAV vector approach can effectively and long-lastingly correct the profound deafness phenotype of these mice when administered well after their auditory system has matured (P30). The present invention therefore concerns a vector system that allows the expression of the full-length Otoferlin polypeptide, or of a functional fragment thereof, in inner hair cells, for use for treating patients suffering from DFNB9 deafness or preventing DFNB9 deafness in patients having DFNB9 mutations, wherein said patients are patients having a developed and mature auditory system, such as new born babies, toddlers, infants, teenagers or adults.