Or a grain of sand....
I think you hit it right on the head. As a sound absorber, it seems like a great solution. Cheap, non-toxic, and non-permanent. But I assume you are talking about absorbing sound so that it doesn't go elsewhere (i.e. through your floor into your family room below, for example). I don't think you'd actually want to absorb sound "in the room" as it were. You just use the volume knob for that! And I would have to agree that it does provide a great, stable, non-resonating platform for speakers.
Is your setup in your basement? If so, there is nothing below it but dirt (assuming you are not in a beach house on stilts).
And if it is in your basement, I am going to assume it has a concrete slab which also would provide a nice stable, non-resonating platform for speakers.
So for me, since my setup is in the basement, I just moved my speakers off the stage and onto the concrete slab.
Now, if you mean that sound absorption as in "tuning" the room, that is a whole 'nother matter. I didn't feel confident that the effects of the specific volume of sand required to fill the stage framing would necessarily equal any particular magic metric that would effectively "tune" a room. But that is probably just my relative lack of sophisticated knowledge in these things.
Then you just have to make sure that the stage doesn't end up having a detrimental effect on the acoustics of the room. I guess I bought into the concept that a sealed box like that could somehow act as a "drum" and resonate. I don't know if there are other possible detrimental effects that a stage could make?
All great things to think about. As we used to say, things that make you go "Hmmmm"....