In some commercial applications the suspended ceiling maybe consider as part of the return airflow or Plenum. This could result in an in ceiling speaker having to be installed in a fire proof enclosure. You need to check your local building code to see if this applies to your installation.

In addition there are some comoanies that sell brackets and enclosures for in ceiling speakers in suspended ceiling.

[Link: fourjay.com]

Disclaimer: I have no experience with this company and their products.