What ever sound comes out of the front of a speaker, a similar amplitude of sound comes out the back of the driver.

Start with a quality back box. We use Triad speakers, which come with a sealed enclosure. (some cheaper rounds do not, but obviously not the right product for a project like this)

You can add insulation above and around the speaker enclosure, or you can have them apply spray-foam insulation to the bottom of the sub-floor above the speaker.

No matter what you do, some sound will probably come through the floor when the system is turned up. Be sure and set those expectations. I have yet to find anything "soundproof" in a house.