I read about this trick a while last summer or fall, and just now finally got around to trying it out.
Materials required:  a cheap fibre fill pillow (i bought one from Walmart – cost $4.97.
Tools required:  a small torx driver, sorry I don’t recall which #
Removed the 3 screws from each speaker in the top sound bar just behind the front seats, and let the speakers hang by the wires.  I then ripped open the pillow, and pulled out chunks of fibre fill, and stuffed it into the holes – not too loose, but not too packed either.  Mounted the speakers and grills bak in place.  Did the same with the rear sub woofer.
Amazing the improvement in sound without spending a small fortune in new electronics!
I didn’t do the front lower dash speakers – but I think I’ll take a look another day and see if the fill would be contained – otherwise, I might just see about some foam cones to mount in behind them.