Simply put, aesthetics is to sight as _________ is to sound. I’ve had this discussion with a few linguiphiles, but to avail.
Why can’t “aesthetics” apply equally to auditory input as well as visual? There’s nothing in the definition that precludes it.
Barring that, my best guess would be “harmony.”
Hmmmm. Point well taken.
Hmmmm. Point well taken. That being said, I also like harmony.
The reason I asked is my son is learning to read employing the methods of phonics. He wants to know why the word answer is pronounced the way it is.
Well, I too like pldennison’s answer. Another good word IMHO is euphony.
how about ‘acoustics’?
I don’t think “acoustics” is really appropriate. It refers more to the scientific principles behind sound transmission and propagation, without regard to its qualities.
“Euphony” is a pretty good answer, actually, better than “harmony.” Especially in light of what prompted the OP. Although questions of how pronunciations evolve are a bit more complex–I bet that at some point in the past, “answer” was pronounced exactly like it looks, just like “knife.”
Mellifluousness?
Mellifluity would be nice if it were a real word; I vote for euphony as well.
Aesthetics is the philosophical study of what makes things pleasing or beautiful. Nothing in the definition of the word, or its etymology, suggests that it is exclusive to the visual arts. I’ve certainly heard the term used with respect to music before, i.e., “the aesthetics of punk rock” (if you can believe such a thing).
I don’t think you need another word.
The other suggestions are for adjectives that describe sound as pleasing, but do not describe the philosophy and/or study of what makes those sounds pleasing (or not - application of the term “aesthetic” does not necessarily imply a positive experience - maybe that’s what they meant about punk rock).
Pedantry:
If you think about roots, the opposite of aesthetic, per Greek, would be ANaesthetic. So maybe something that is aesthetic simply makes you feel somehow (not necessarily nice-- which is why very icky art is still aesthetic. Aesthetically PLEASING is a different matter).
I like euphony in your context. Has a nice ring to it 
I second (or third, or fourth) **scratch1300’**s answer: euphony or euphonics