Hi @caidom ,
Have you verified that the problem is in the actual headphones and not in the cable or plug?
Plug the headset into an audio output source, firmly hold the plug into the socket of the audio device and then gently move the cable where it exits the plug, back and forth and listen if the sound comes and goes in the left earphone. (only "wear' the left earphone to make it easier to discern if it is there or not).
If you still cannot hear anything try the same thing where the cable enters the headphones. Gently move the cable back and forth and listen if the sound comes and goes.
A lot of times the cable fractures (internally) where it exits the plug because people generally unplug the headphones by pulling on the cable and not the plug.
Also if the problem appears to be at the plug end it may be that the plug connection for the left speaker is loose or faulty. The left speaker is connected to the tip connector in the plug.