Why are Earpiece's magnetised?

Answer 1

The simplest form of transducer from an electric signal to sound waves is by way of a small electro-dynamic transducer - really a tiny loudspeaker. But of course any transducer between these energy types can be used. Piezoelectric, capacitance, and so on.

Your doctor's stethoscope does not have magnetic earpieces, nor do the earpieces used to communicate with the patient in CT or MRI scans.

In the latter case, the metal would seriously interfere with the diagnostic process, and the solution is to have the audio transducer remote, and to connect that to the patients earpiece with plastic tubing. Quite like the speaking tubes on older ships - simple but reliable.

Answer 2

Earpieces in headphones or hearing aids are magnetized to convert electrical signals into sound. The electrical current passing through the voice coil creates a magnetic field that interacts with the permanent magnet in the earpiece, causing it to push and pull the diaphragm, creating sound waves that we can hear.