I'd like to point you to a past post of mine regarding heightened senses, you may be able to get something out of it.
Basically I started heightening my psychic senses and as I did that my regular senses also 'picked up', so they may work hand in hand, heighten one and the other follows...
Super senses
And regarding Synesthesia I would like to point you to this article, it is what I had been practicing (as mentioned in the above link) for my sense of hearing. Can't comment too much on my hearing though, whether it is any better or not.
Sound Meditation