The same channels , that create the deadly prolonged QT interval by delaying the repolarisation in the heart is responsible in the for the deafness as it interfere with inner ear
Mechanotransduction of sound into neural signals .
For proper auditory function , the cochlear hair cells needs a continuous flow of endolymph which maintain a voltage gradient for nerve signal transmission .The lymph secretion is is regulated by potassium channels KCNQ1 and KCNE1 . Mutations of this gene impairs the K + content of the endolymph. It results in a compromised endocochlear potential (Difference between peri lymph and endolymph potenial ) .This result in irreversible deafness .
Link to a good illustration from Medical physiology