[knob]

You may be able to fix it without replacing the jack.

If you open your player your may be able to get at the plated brass strips that make contact with your earphone plug.

Assuming you have your player opened up all you need to do is with the earphone plug removed press down gently on the strips on the socket with a small screwdriver to make it a tighter fit for the plug.

What usually happens is if you unplug the earphones a lot the connection will get loose, I try to avoid unpluging the earphones unless I really need to to save wear and tear on the socket.