Ear is a fascinating piece of machinery with interesting structural details and functions. It is an interesting topic to study since, just like vision, it is something you use every day without giving it much thought.
Music has been a hobby of mine for 17 years and still I never think of how the sounds I play cause vibrations that are transferred through mechanical systems to nerve impulses and perceived by the brain. When you realize how many small structural parts like the oscillators etc. it takes to create the sensation of hearing, it is easier to appreciate it. A good reason to remember to protect your hearing when necessary.
It is interesting how different sound and harmonies are connected with feelings. Like how can you say that a chord is in a minor when it sounds sad? Also the mixture of sounds and physical feelings is interesting. Most people have experienced that loud thump of a bass drum that makes your whole body tremble. This is also a way for the deaf to enjoy music. Could this be used in some kind of device to provide a larger scale of physical feelings in relation to different pitches. Also automatic reactions are interesting, why some high pitched sounds make the hair in your arms to stand up and give you shivers?
Sound and hearing related synesthesias would be an interesting topic of study. Like how people with auditory-tactile synesthesia associate certain sounds with certain physical sensations without actual touching. Or how in lexical-gustatory synesthesia, hearing a word evokes a taste sensation. How is this caused in the nervous system?