I read in a few places that in a guitar for example, the vibrations are passed through the connectors to the wood and the wood with its bigger surface is more efficient as a coupler to the air, resulting in a stronger sound (and, therefore, the vibrations should decay more quickly. Also the wood's dissipation enters here). However, one can amplify, let's say, ones smartphone's playback, by putting it in a box or near one. In this case the vibrations are excited in the air, which excites them in the box, which excites them back in air. One could expect a reduced sound due to the air-box-air excitations inefficiency. How come it ends up with a louder sound? How, and where is the energy from?
if you place a noise source into a chamber with little acoustic damping in it, then the energy being radiated by the noise source builds up inside the chamber with time and can exceed the loudness of the original source if it were playing into free space.
This principle of amplification by buildup was used in stone cathedrals to make voices louder, but it also impairs intelligibility of the resulting speech because the syllables of the words get smeared together by all the acoustic energy bouncing around within the space. Priests had to speak s l o w l y to be understood because of this reverberation time effect.
My opinion: If a sound source (Smartphone speaker or guitar string) is smaller than the sound wavelength, it is very inefficient in radiating the sound. 99% of the energy is wasted in local interference and only 1% go to long-distance waves. Smartphone with their tiny speaker are good at treble and bad at basses; while bass speakers are built as large as possible. (100 Hertz sound -> 3,4 metres wavelength!)
The cone of a loudspeaker/grammophone is the first trick to harvest the energy from a small source by making it bigger, thus avoiding the interference; it also directs the sound. The energy conversion from vibration to long-distance sound is more efficient and sound is "amplified".
The soundbox combines this trick with two more features:
It accumulate vibration modes in it, somehow coupled to the source.
(Guitar & violins) The small vibrations of large walls lead to change of volume of the cavity, which is released by large movements of air (sound source) forced through the small sound hole.