I kinda agree with Jonny. Its not that its uses are limited. But, the knowledge to use them, and likewise the demand will be limited. The spheres are very nice for making a lightweight epoxy filler, joining foam blocks for core construction, fairing and void filling where the ultimate in material performance is required. Spheres are needed and very useful in airplane, boat, and car parts. However, they would be of little use in car audio.As far as I can tell, most still use poly over epoxy in any case. If filler is needed, it is likely that some poly filler would be used.
With that said, I have tried to employ various advanced composite techniques (using core materials) for building my dash sub box and kick speakers. I have yet to find anyone who has tried the same. Because of the extremely asymmetric design of the dash sub box, and the amount of piecing together and modifying I have had to do, microspheres would be very handy, but not absolutely needed. Very steep learning curve, and lots of experimenting and failure. In all honesty, I don't think that most people in their right minds would ever try what I am in any case :laugh:.
Plus, microspheres are sehr nasty to handle. If you have a misfitting respirator, fumes may give you a nasty headache. But in the same situation, glass spheres lodged in your lungs can cause permanent damage.
I can see where fillers would be very nice for making adhesive pastes, but other fillers can be used with adequate results.