How can Brook see and hear without organs? How is it possible that his five senses are still working?
How can he even talk?
In short it is because of the Yomi Yomi no Mi Fruit's power he is able see, taste and do other things.

According to wiki:


  Through the fruit's powers, Brook is able to function without the
  presence of necessary and vital body parts that have decomposed long
  ago. He is able to interact with the environment around him without
  the presence of eye balls or any other sensory organ to aid him. He,
  in particular, retains his ability of speech despite his vocal chords
  and even his windpipe have long since rotted away. He is able to taste
  and digest food like any other normal living thing to sustain himself.
  Drinking milk in particular heals him as the calcium intake repairs
  any of Brook's bones that may have been broken in battle. Depending on
  the situation, Brook is also able to produce organic matter such as
  tears, fecal matter, and even blood from his nose despite not having
  any organs nor bodily liquids to produce such matter.