Make sure the headphone plug is fully inserted into the iPod's headphones jack.
Also, try a different set of headphones. It's possible that your usual headphones have become faulty.
The iPod has "equalizer" settings, so make sure it did not get set to something that makes it sound "tinny." The setting is on the iPod's Settings screen; it may be under Playback.
If that's not it, you can try doing a Restore on the iPod, in case the cause is corrupted software or settings. The Restore button is on the iPod's Summary screen in iTunes. This will erase the iPod, reinstall its software, and set it to default settings.