Concrete Audio Speakers...Speaker Enclosures made out of Concrete ?..

Concrete, granite, cast iron... Companies try, but really, how practical is this?

Its not a question of practicality. They sound bad. 

I once saw a demonstration about concrete boat hulls. At one point the speaker struck the side of the concrete hull (it was an unfinished boat out of the water) with a pipe or bar and it rang like a giant bell. We actually sailed a concrete hull sailboat. Beautiful boat.

To say one material is always superior to another and another material is always inferior is an over simplification of what's involved in the designing of any product. The implementation is just as important as the material used.

For example, some people here argue that aluminum is the best material to use for speaker enclosures. Which alloy are you referring to? 6061, 2024, 7075...? What cross section? What shape? Pick up a piece of 0.5" thick 7075 and strike it with a bar, it will ring big time. I'm not saying aluminum isn't good for use in speaker enclosures, it can be great if used properly. BTW, properly designed aluminum extrusions can be economical and practical for speaker enclosures if you sell enough product.

I tried concrete, and the sound was weighty and too hard. I wanted something more airy. So now trying styrofoam.

BAD idea it will sound like bricks  are you joking?