In case you've faced the issue with playing back your files on an iPod, you can consider one of the workarounds below:
You might need to 'flash mod' your iPod (to replace the hard drive with a micro SD card in a couple adapters, so it's faster, more reliable, and less taxing on the battery).
If you add more than 2-3K files at a time, this might happen. Just delete them and readd the songs. Consider transferring fewer songs at a time.
The issue might be in the file/files itself—it could be corrupt.