Try:
- Reset the iOS device. Nothing will be lost
Reset iOS device: Hold down the On/Off button and the Home button at the same time for at
least ten seconds, until the Apple logo appears.
- Unsync/delete all music and resync
To delete all music go to Settings>General>Usage>Storage>Manage Storage>Music>Tap edit in upper right and then tap the minus sign by All Music
- Reset all settings
Go to Settings > General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Information (such as contacts and calendars) and media (such as songs and videos) aren’t affected.
- Restore from backup. See:
Restore your device from an iCloud or iTunes backup - Apple Support
- Restore to factory settings/new iOS device.
I suspect a larger "other"
OTHER
Connect the iPod to your computer and look at the storage used by the various categories as shown in the colored bargarph in iTunes.
Come back with the results.
An "other" larger than about 1 1/2 GB indicates corrupted files.
What is the Other on my iPhone and How to Remove It
What is "Other" and What Can I Do About It?
Next, usually restoring from backup eliminated the corrupted files. However, sometimes restoring to factory settings/new iPod is required.
To restore from backup see:
iOS: How to back up
To restore to factory settings/new iPod see:
iTunes: Restoring iOS software