How to record audio off an iPhone 4?

I have an iPhone 4, and I want to record the audio played on the phone and save it on an mp3 and put it on my computer. Anybody have any ideas on how to do this?

All of our audio downloads are MP3 files and can be played on your iPod, iPhone, Android phone, and on any portable audio devices, tablets, or computers. All downloaded MP3 audio files can be transferred via your computer to your mobile smartphone utilizing iTunes for the iPhone, or the Music folder of your Android operating-system on Android phones