Hello there, stan20.
The following Knowledge Base article offers some great steps for troubleshooting your issue:
1. Verify that there's no debris in the headphone port.
2. Connect and then disconnect the headphones.
3. Verify that the headphone connector is pushed in all the way, so that the headphone's plug shell is flush with the headphone port, like this:
4. Try resetting the iPod:
- Toggle the Hold switch on and off.
- Press and hold the menu and center buttons simultaneously until the Apple logo appears.
- If you need assistance with resetting your iPod, see this article.
5. Try another set of Apple headphones.
6. If you have tried all the previous steps and you still hear sound coming from the speaker when the headphones are connected, your iPod may need service.
Thanks for reaching out to Apple Support Communities.
- It might seem obvious, but have you made sure the volume control is not turned down?
- Have you also made sure that the Volume Limit is not turned down? Look in Settings/Volume Limit and make sure it is not at the bottom of the scale.
- I know you said you have tried a second pair of earphones, but you have made sure that at least one of those pairs works on other devices, haven't you?
- Have you checked the headphone port to make sure there is no debris in there (as suggested)?
- Have you tried a Reset, as suggested?
To do a Reset, place the iPod (Classic) on a flat surface (where it's easier to do the following);
- Turn the Hold switch on, then off again
- Press-and-hold both the Menu and the Select (centre) buttons at the same time for at least ten seconds. At some stage, the iPod should re-start and you can let go of the buttons. After it re-starts, it will have reset.
After a Reset, is the problem still evident? If so, perhaps there is a problem with the iPod. In that event, try making an appointment with an Apple Genius (in an Apple Store) to see if they can help you.