Hi
It might be a bad battery causing the issue but it could be that you are using headphones or speakers that have too high a impedance (ohms)
Basically the more ohms something has the harder it will be to make it sound good and get loud
To check if it’s a impedance issue use the apple headphones that come in the box to see if the issue persists
If you charge the iPods up and see how long they last that should give us a indication of the battery’s health
If you do decide to open them look out if any damage,discolouration or corrosion as it can cause these sorts of issues
Hopefully this helps
Any questions please ask
Thanks:-)