So, I'm not sure how many people have random broken radios/keyboards/CD players lying around, but if anybody's got a bit of time and a couple random items, you can turn them into this piece! It doesn't get very loud, but you don't need to plug it in - I take it outside while sunning in the summer to listen to my music. Plus, you get to design the box yourself!
Take apart your CD player/piano keyboard/radio. I personally used an old broken keyboard I found on the curb on garbage day. Taking it apart can take some patience and time, depending on what sort of casing you're dealing with. Your main goal is to find the speaker and remove it. For my keyboard, it was just a matter of taking off the back and removing some screws to free the speaker inside. There'll be some wires attached to the speaker, try to keep those attached to the speaker, you'll be needing those!
Depending on the size of your speaker, the size of the cardboard box you'll need may vary. I used a box of instant oatmeal packets; it was roughly the same width as the speaker. Place the speaker face down on the front of the box and trace around it. This will give you a rough idea of the size of your speaker and how much of the box you'll need to cut.
Carefully tear open the box - usually you can see where the box was glued together in manufacturing; those places are the easiest to tear it apart from. Then, cut out the hole you drew on the box! You should try to leave an allowance of 1 centimeter or so inside the hole so that your speaker isn't going to fall out.
Place your speaker face-down on a piece of fabric (I chose red felt) and trace around it. Cut this piece out, and then glue it inside the back of the box to cover up the hole you just cut out.
Now, take your cord and cut it, leaving a length of roughly 1.5-2 feet from the little headphone jack. I used a mono-to-mono cord, but I imagine you could have luck using just the end of an old headphone cord. Peel back the rubber to show the wires.
Now, remember the wires that are attached to your speaker? Wrap the ends of the wires from the speaker together, and then wrap them around the wires inside your mono-to-mono cord (or headphone cord, or whatever). Use a bit of electrical tape to hold the wires in place and protect them from falling out. (During this stage, it's a good idea to have your mp3 player handy so you can test it and make sure the wiring is okay and that it'll work!)
Tape your speaker inside the box in place behind the hole. On the back of the box, cut a tiny hole near the base, and feed the cord through the hole. (Try to keep the taped-up part of the wires inside the box, it looks much neater that way.)
Now, you can begin to tape your box back together! (You're almost done!) Once you've taped the sides together, stuff newspaper into the box from the top and the bottom to make sure that if the tape comes off inside, the speaker will be held in place by the paper. Tape up the bottom and the top, and now, paint it however you like! Voila!