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 :/.

>! Euler's number is used to approximate limits

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).

>! ??

Beat that! I'm in between the lines of music that have a very troublesome chef.... just below the highest "seen" staff. 

>! In bass clef, the note "e" is inside the staff.

Look at that buns I just made! (I know it's bad)

>! ??

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

>! ??

A child uses me when in digust (sometimes).

>! **e**w?

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

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