As per requested...

(This is an excerpt from the book "Appreciating Navadvipa Dhama" by HH Mahanidhi Swami)

The following popular story explains the appearance of the mridanga in Gaura-lilä: Before coming to this world as the Golden Avatära, Lord Krsna said to His flute, “My dear flute, I am descending again to the material world but not in this form. You should stay here with Rädharani to pacify Her. I promised Rädhä that I would not take you with Me.”

On hearing this, the flute began to cry and said, “My dear Lord Krsna, how can You leave me here alone. I have always been by Your side, but now You are neglecting me. What have I done to deserve this?”

Krsnaa replied compassionately that He would bring him along but in a different form. The flute did not mind changing his form, as long as he could remain engaged in Krsna’s personal service. In this way the flute transformed into a mridanga, and descended to this world with Sri Caitanya Mahäprabhu. 

            ASA -  Uncle Gismo:    Hmmmm!    Popular story.       Not very scientific. (But cute... :O})       Lord Caitany doesn't play Kartals of Mrdanga in His lila. Rama has a bow, Krsna has his flute.            Hmmm...   Thank you for the news.   We have some more ideas to send....    Soon!