。
Blind people often have sharp hearing.
Literal
Eye [subject-が] not-seeing person [possessive-の] hearing-ability [topic-は] sharp case [subject-が] is-many.
目が見えない ('eyes that do not see') acts as a relative clause modifying 人 — 'a person whose eyes do not see,' i.e. a blind person. The structure 鋭敏な場合が多い ('cases of being sharp are many') is how Japanese hedges generalisations: instead of 'often X is Y,' it says 'there are many cases of X being Y.' 聴力 is 'hearing ability' — 聴 ('listen') + 力 ('power').