The headphones from my old Xperia Ray turned out to be incompatible with my new Xperia M. They are still fine, but I can't use them with the new phone.
Also they can't be used with a simple MP3 player as the TRRS jack passes the audio output through the microphone, so it's barely audible.
There's the 'press mute button' workaround, but it's quite user unfriendly to keep holding it while cycling...
So, to fix them permanently, I opened the microphone enclosure (a fingernail works well).
You'll notice the two wires marked G and R that connect the mute button.
When shorted, the effect is the same as if the mute button was pressed permanently. And that's what I've done.
A blip of the solder gun and the headphones can be used!