Try lubricating the extension and the plastic piece holding the pad that cushions the top of your head and see if that helps the adjustment slide. If not, it might be a mechanical problem. You can check out the slide adjustment repair guide and hopefully it works!
Check that you're not muted on your computer. If that doesn't fix it, you can also check if the controller is faulty by testing the microphone with another program. Is the problem is still there, you probably have to replace the microphone jack <LINK HERE>.
Make sure that the headphone is fully plugged into the audio jack, you'll hear a light click if it's properly done. If you think the speaker cone needs to be replaced, check out the speaker cone replacement guide.
