In case of Android 4.0.4 / 4.1 you should do the following steps:
Settings → Apps → All. Locate the following:
- ProtectedMediaScanner → Clear data & Force stop
- ProtectedMediaScannerTrigger → Clear data & Force stop
- Media Storage → Clear data & Force stop
- 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.