A good way to quiet the Miata...
I got this suggestion from CraiginTX and it made a HUGE difference so I thought I'd share.
Get some pink home insulation, place it in two plastic bags and seal them up with some tape. Now stuff a bag on either side of the gas tank under the rear deck.
This actually made more of a difference in my car than the sound deadener I applied. ::Rant::
So much so that I actually heard my headrest speaker much better. The car is significantly quieter and for you weight conscious folks, this adds very little weight.