Seems like amazon releases free mp3 codes around product launches. They just announded the "fire" tablet today. They have another $2 credit for mp3's out there too.
Go to this amazon link - http://www.amazon.com/gp/feature.html?docId=1000296831.
Click on "Enter Your Code".
As easy as that - $2 credit for mp3 downloads. That is 2 free songs, since each song is usuall $0.99.