I second RTROSE's suggestion.
1 - Get a piece of drywall, or other thin/light plywood type material, and cut it to the size of a tile.
2 - Glue one square of drop ceiling to the "bottom" of the drywall or whatever you chose.
3 - Secure speaker mount to your chosen material, through the ceiling tile.
Using this method will make this tile look like the rest, but it will be strong enough to hold the weight you need. You will probably need to reinforce the metal runners that this tile sits in with a couple more wire supports.
I think I would try this with drywall first, combined with molly/toggle bolts.