The word to put into the blank is "hearing loss".
Beethoven famously lost his hearing in the middle of his life, and his hearing loss was gradual and started when he was in his 20s.
Despite the hearing loss, Beethoven continued to work on his music, and composed many of his most valued works when he was already deaf.
The Heiligenstadt refers to a place where the recipient of the testament lived, a village which is today in Vienna.