You are

>! e 

I'm a letter.

>! E is a letter

I am found in an elephant 

>! E is in the word elephant

and oh! I am heard too, by a person sense of hearing!

>! **e**ar

Sometimes, I am also used in a cartesian plane, or maybe used as a word to describe things are growing really.. really fast.. unless I am negative :/.

>! **E**uclidean plane (thanks to @Dalton) and **E**xponential, also e^x?

I am a letter, but found commonly in letters! 

>! E is the most commonly used letter in the English language

And I am also sometimes commonly known as a latter (In English).

>! **e**nd

Beat that! I'm in between the lines of music that have a very troubled chef.... just below the highest "seen" staff. Look at that buns I just made! (I know it's bad)

>! e is the highest note inside the staff on treble clef (pun time!!)

I'm used in banks after I was discovered by approaching to a limit. I mean, I was discovered to have numeric use.

>! Honestly still stumped by this one

A child uses me when in digust (sometimes).

>! **e**w?

Oh... I'm so rational... NOT.

>! e, as in euler's number, is irrational