Wonderful old expression for a large, bronze cash register. It has many keys on the front. When you press a key, it "rings up" the amount you pressed, and it rings a bell. You have to press several keys to "ring up" a large amount of money. Each of these rings is beautiful music to any merchant.

When you finish the "song," the Jewish Piano opens and you can put in all that wonderful money you just collected.

A cash register. Specifically, a large, old-fashion bronze cash register that rings bells when you press the keys.

From the proverbial, stereotypical notion that the sound of money is music to a Jew's ears.