You'll need one star jeweller's screwdriver and two flat jeweller's screwdrivers.
Please bear in mind that following these steps *will* invalidate any warrantees, guarantees or any other promises/agreements relating to the device. You are also quite likely to break the device, set your house on fire and howl at the moon in frustration. If you find all of this acceptable, please carry on!''
Step 1: Remove the Hold Slider
Step 2: Remove the Front Cover Screw
Step 3: Remove Front Cover
Step 4: Remove Board Cover Screws
Step 5: Remove Board Cover
Step 6: Remove Top Board Screws
Step 7: Remove Top Board
Once the ribbon cable has been removed, pull the right-hand side of the top board gently towards you using a flat-head screwdriver. This will pop a connector on the underside of the top board out of a small socket on the bottom board. Gently lift the right-hand side of the top board up over the lip of the headphone connector and then you can gently slide out the top board towards the right-hand-side, bearing in mind that the charge/data connector is quite tightly fitted in the case. Also, be very careful that the battery cable on the underside of the board doesn't snap - remove the top board and the battery at the same time.
Step 8: Remove Bottom Board Screws
Step 9: Remove the Bottom Board
Remove the headphone jack ribbon cable by lifting the black tab on the connector and gently pulling out the cable. As with the LCD cable, folding, bending or ripping this cable means that it will stop working!
You can now remove the bottom board, bearing in mind that there is a small piece of double-sided tape holding the one side down - gently remove this.
Now is a great time to find out how many screws you have lost. They're tiny and for some reason very prone to bouncing away. ;)
Step 10: Fixing the Problem
It's entirely possible that by disassembling and then reassembling the device, you problem will go away! This is because the headphone jack connector/cable seems to oxidise ('rust') and removing the cable causes most of this 'rust' to be removed. This is what caused my issues.
I fixed the problem by gently scraping off the 'rust' with a flat-head screwdriver. It's a _very_ tiny connector and even with a magnifying glass, I struggled to see what I was doing.
The alternative is that the headphone jack itself has broken. I spent an hour or two trying to disassemble the headphone jack and didn't get anywhere, so if anyone has some hints, please drop them ;)
Step 11: Putting It Back Together
It's quite simple to put back together - just follow the steps in reverse - nothing complicated.
Enjoy and good luck!