Please help :( 3.) What is the physical quantity which
represents the basis of the functioning of a
loudspeaker?
4.) Why doesn't a direct current operate a
loudspeaker?

3. A loudspeaker works because of the “motor effect” meaning electricity and a magnetic field cause movement.

4. A direct current will cause momentary movement in one direction only. For the speaker to produce sound you would need alternating current which makes the cone vibrate back & forth.