One way is to cut a piece of 3/4" plywood to fit the tile opening and lag the mount up through the tile into the plywood. If you would choose this method you should have a couple of steel cables anchored into the hard deck and the other ends attached to eyelets into the wood for safety and some strain releif of the tile grid.

Another way is to mount a pipe clamp to the hard deck above the drop ceiling and have the pipe poke down below the tile and mount to the pipe.

Whatever method you choose, the speaker should be mounted in a way that it stays put even if the grid were to come down. At least that is the way they see it here at the University.

Matt