What amplifies sounds?

A sound amplifier. Answer. While electronic amplifiers will amplify sound (we do this all the time), there are also mechanical devices that "direct" the sound and make it sound louder. They don't...

..frequency of sound is equal to the eigenfrequency (which depends on the shape of the guitar).

Hard reflexions from walls reflect the sound and add it to the direct sound.That gives amplification of the total sound.

The sound when sent out of the speaker (big or small) hits the walls of the paper cone causing this sound to keep hitting the walls time after time makes the sound hit its self and there for makes a...

Yes!