the back connector is no doubt soldered straight onto the motherboard, but the front connector is probably a separate board on a fly lead which would be easy to change. I was going to see if I could track the part down for you.
If it's got a front and back speaker connection then the chances are there is not a separate sound card, it will be built onto the main board, which leaves you a couple of options.
replace the front panel connector and carry one using the sound card built into the machine.
Add an additional sound card (assuming the computer has a spare expansion slot inside it)
TBH, cost wise there probably wouldn't be much to choose between the options, they should both be pretty cheap.
It seems to me unlikely that the rear socket would fail, followed shortly by the front socket. The common denominator is the plug itself. I would suspect the speaker lead and jack before considering anything else
Thanks everyone. I called my go-to guy and he talked me through an unistall - reinstall of the necessary bits, and it works fine now - I am dead chuffed!