Yep is the word you use for when you want to use sarcasm act like you heard the person who asked you a question.
Examples would be: "You cleaned your room didn't you?"
Since you were listening to music and didn't hear your mom, you say: "Yep.'
Another example: "Did you turn in that paper at school that will result in you failing if you don't?"
It is a common usage for southerners, even though they don't use it.