Nokia Music Unlimited is only available in a few territories, and much like a one year Zune Pass, offers unlimited musical downloads from the Nokia Music Store to your handset. It will only be visible if it is available in your region. The alterations to the Nokia Music Player (see our second screenshot) add a shuffle and repeat button to the interface, adding in the controls that are present in the built in music player.
Unlike some of their other Windows Phones apps, the Nokia Music application is only available to Lumia owners. You can get the update (or redownload the app) from either the Windows Marketplace, Nokia's App Highlights, or from the QR code in the AAWP App Directory.