Attach the speaker where it will be set during the performance. Usually, it is set facing out into the audience. Make sure the box is out of the way and won't be tripped on or bumped into by any actors.
Take a thin piece of black material, trimmed to approximately three times the speaker box, and drape it over the box. Pull the material tight on the part that is over the speaker itself. Staple the material to the sides of the speaker box, keeping the material taut over the speaker.
Mount the speaker, leaving the material hanging. It is especially important to leave the back of the speaker uncovered, as this is where the wires go in and how the speaker gets mounted.
Using the black material, cover the edges and frame of the speaker box. Attach the loose edges of the material to the surrounding areas, such as on the floor or against the wall. The material should make the speaker box "disappear" on stage.
Blend the black material with other colors and/or items, if necessary. This will cover the speaker box even more and blend it into the set or stage.