<p>My goal is to have a speaker inside a PLA casing to produce a nice hearable sound from a submersible item.</p>
<p>I intended to produce that sound from a Piezo buzzer stuck on a membrane held tight in hollow place between two pieces, but the result is unsatisfactory as the sound gets muffled to inaudible levels.</p>
<p>Is there any known 3D printable methods to permit for a sound to be heard from PLA?</p>
<p><a href="https://i.stack.imgur.com/TLaB3.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TLaB3.jpg" alt="enter image description here" /></a></p>

In space, no one can hear you scream. That's because there's no air to be vibrated, which is part of the definition of sound.
In the case of your model, the described diaphragm will generate sound because it is surrounded by air. The vibrations in the air will transfer (and reflect) from the PLA shell you've created.
In order to hear as much sound as possible, you have to have as thin a shell as possible and only one of them. It would be more effective if you could incorporate the membrane into the shell, which eliminates the sound damping effect of the plastic sphere.
Barring that option, one layer (vase mode, but probably not) would present the created sound with a much less massive amount of plastic to vibrate.
Additionally, PLA is not known for being water tight. One can apply epoxy or other sealants to make it so, but that will add mass to the overall equation.