If you are inquiring about the early sound reproduction machines, such as the one featured in the famous picture of a dog looking into the horn of a phonpgraph, where there is no electrical amplification, then what is being used to amplify the acoustics is horn technology. You might be familiar with megaphones used by cheerleaders and those long horn type tubes used at soccor games - I forget the name of those things, and the bullhorn used at public address systems. Even the high frequency speaker or tweeter uses horn technology ( as does the woofer but there the absence of a noticible horn is the main feature )
The horn works by acoustic impedance matching. The sound reproduced at the narrow end is limited to a small area which gives a high impedance ( ie higher pressure ) for the reproduction. And as the sound moves through the horn it spreads out, and the wave pressure drops. If you remove the horn assembly and just try to listen to the needle scratching the surface, you will get an idea of why impedance matching is important.