(Note: Although unhear is not a true word in the dictionary, it will be used throughout this post for a relevant comparison to the term unsee)
Without being a scientist or neurologist, we can all pretty much conclude that the brain has the power to retain and store information. This conclusion is evident based on the experiences of each and every one of us. Once the brain processes the information that is either heard or seen, it is sometimes hard to "un" any of it: unsee or unhear.
In many cases, the ability to unsee or unhear is considered to be great especially when it proves to be pleasant or beneficial. When comparing, which do you think is harder to deal with when it's unpleasant? Things you can't unsee or Things you can't unhear? Your choice will most likely depend on your experience.
In my opinion, I think both are hard, but it's something about unhearing that plays in your head like a song. Not just any song, like your favorite song. Of course, it isn't really your favorite song, but the jingle of the spoken words that may have hurt to the core is something that becomes hard to unhear. So how can you unhear something that has hurt you? Simple, but may take some work. Drown them out with spoken words that do the exact opposite.