DKKA wrote:
Of course I have.
Oaky, well now we've ruled out the headphones (which needed to happen), it does seem likely that the headphone port's contacts have failed. A common cause of this is the headphone lead being wrapped tightly round the iPod with the headphones still plugged in. If the headphone plug is the long type, and it gets pushed sideways in relation to the iPod itself, this can create a tension between the headphone socket and the board that it's mounted on and the result can be broken contact between the two and loss of one or both channels.
So that perhaps explains what may have happened. What can you do about it? Well, possibly you were lucky with the first replacement, since this would normally be described as user damage which is not covered by warranty. I would expect though, that you could use the Apple out-of-warranty service, where you pay for the repair. Alternatively, try a third party repairer.
For Apple, start here: https://selfsolve.apple.com/agreementWarrantyDynamic.do