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If you look at a typical headphone / earbud jack, you will see it is 3 pieces of metal separated by two plastic insulating rings. The one nearest the wire is common; the tip and the middle are left/right (not sure which way around). Use a file or a rotary tool to grind a little bit into the gap with the insulating ring between the left/right sections i.e. the tip and the middle section. Then dab a bit of solder to join them. Not too much so it does not interfere with it sliding in and out of jack sockets. The headphones / earbuds will now play mono.