One speaker works when the cable is plugged into opposite side of head

Hi

I think your headphones have a break in the cable somewhere as a break can cause this issue

It could be a board level issue but it’s very very rare and cable issues are much more common that them

Here’s a video on how to replace the cable

https://m.youtube.com/watch?v=c06x-MdN_Z...

this video show you how to replace the headband

Once you have your new cable for the inside you will have to swap the cables out (new one being used)and reassemble normally

Just as a heads up it will require soldering but it’s not too hard:-)

If you need any help along the way please do not hesitate to ask

Thanks:-)