Remove the speakers from the cabinet, if applicable. Cabinet styles vary, so check the unit's manual. Screws are commonly located on the front and can be removed with a screwdriver, allowing the speaker horn to be pulled out.
Clean the cabinet thoroughly with a slightly damp cloth. All-purpose spray cleaner can be used on most surfaces. Just be sure to wipe it dry immediately, especially on wood.
Cover any part of the cabinet that you do not want painted with paper, using masking tape to hold it in place and to provide a neat edge. If the speaker cabinet has a non-removable cloth grille, cover it completely.
Trim the edges of the masked area with a utility knife so the lines are clean.
Spray the exposed area with primer, making sure the area is well-ventilated and that the surrounding surfaces are covered in newspaper. If you will be painting the cabinet a dark color, use a tinted primer. Dry completely, and repeat if necessary.
Paint over the dry primer coat with spray enamel. Allow to dry completely, and spray a second coat if necessary. Let that coat dry as well.
Remove the tape and paper. Re-install the speakers, if applicable.