In the furniture catalog, there's a model named Box in the Miscellaneous category.

Drag and drop it on your plan. Then, as this box is 1m x 1m x 1m big, resize it to the size of a speaker with the corners of the object, or by double-clicking on it and set its width / depth / height. Finally place it along the wall you want.
For the other speakers, you can copy paste the one you already resized in the plan.