Pyramid Solar-E music box, acrylic with black base. $29.95. Also, 4-inch plastic cube mounted on 4 1/2-inch hardwood base, $26.95, and 3 3/4-inche-by-2 1/2-inch clear plastic cube, $24.95. Indicate alternate tunes. Delivery in two to four weeks. El Tesoro, Inc., Box 197, Indian Head, Md. 20640. 301/843-1998.

It's almost like magic: the rays of the sun strike the little acrylic box and voila! Music! No keys to wind, no batteries, nothing to plug in. This is a solar music box, equipped with a silicon solar cell and a special low-voltage motor. When the sun shines, the music plays. On a cloudy day, set the music box about six to 10 inches from another source of light such as a 100- or 150-watt bulb. (Experiments: As the sun moves across the sky, the motor will slow and finally stop. The closer you are to the equator, the longer and faster the motor will run. Position of the sun, shade from branches and building, cloud covers and other conditions will affect the play.) Music includes "Edelweiss,' "Lara's Theme," "Sunrise, Sunset," "You Are My Sunshine," "San Francisco," and "I'd Like to Teach the World to Sing." Each box includes a summary of the principles and history of solar energy.