Desoldering speaker cable in order to replace with fresh and longer wire
if you aren't comfortable soldering all the wires off, I would suggest you cut the wires leaving about ~5 cm. strip the insulation, clean the wire with alcohol covered swaps, let it dry then "braid" the new wire around the old and solder it together, then cover the splice with some heat shrink tube.

There are many guides out there as how to best splice multi core cables.