When you trade with a villager and e.g. he will give you one apple for one emerald. if you give him 2 emeralds or more and get the stack of apples from him, he will say the agree sounds 2 times at once and so on depending on the amount of emeralds you have.

steps to reproduce:
1. find a villager and trade with him.
2. give yourself 64 stacks of the items he wants.
3. shift click or click on the item he will give you.