Check if you have stored music in the phone memory, that is, the C drive of your phone. Music tracks in C:\Data folder are backed up and restored to your phone during the software update to Nokia Belle. But if you have stored music tracks in any other folders in the C drive, they are lost during the software update. If you have DRM protected music tracks in your phone, note that you lose the licenses during the software update to Nokia Belle. After the update, you need to relicense all the copy protected music tracks you have on the phone.