In case of Android 4.0.4 / 4.1 you should do the following steps:

1. Settings → Apps → All. Locate the following:

 * ProtectedMediaScanner → Clear data & Force stop
 * ProtectedMediaScannerTrigger → Clear data & Force stop
 * Media Storage → Clear data & Force stop
2. Restart your Android phone. Turn it off, then on again. Let it rest for 10-15 minutes. Afterwards your media files should be available by Gallery and Music player.