All you'll need to do this project is:
1. One (or two) old free standing speakers, the bigger the better. Preferably in good aesthetic condition, preferably broken, and preferably free (I gave myself a $20 spending limit for mine).
2. A hand saw
3. Some hinges
4. A claw hammer or crowbar
5. Some paint
6. Planks of wood (to make shelves)
7. Screwdriver or a power drill
8. Roughly 2.5 liters of pure, unadulterated gumption. Feel free to replace with true grit.
I also used a rotary tool, but I'm sure you could pull this project off without one.
Step 1: Disassembly
Step 2: Disassembly (cont)
Fortunately, these are usually only held on by some small metal pins and a lot of glue (as you can see in the second photo), so they're pretty easy to get off. Just take a crowbar or a chisel and hammer it a ways down into the seam between the speaker cone and the magnets, then pull it out and repeat that action around the full circle of the magnet until it eventually pops off (once you get the hang of it each one should only take you 30 seconds or so, unless the speakers are really tough).
Once you've removed the magnets from all of your speakers, wallow in the delicious scent of self satisfaction before returning to work. It's now time to focus our work on the cabinet.
Step 3: Cutting a door
First measure out and mark the lines of your cuts, then make those cuts. To make the awkward cuts on the side of my speaker box I first took a rotary tool and cut a groove into the lines where I was going to make my cuts (pictured), then took a hand saw and finished those cuts. You could probably use a jigsaw or a circular saw to accomplish this same end, but I'm not very good with those tools, so I took the old fashioned route.
Step 4: Internal clean up
Speaker boxes are held together with goopy epoxy-like glues and their insides are never intended to be viewed by anyone, so you'll probably find a lot of glue globs and what not stuck to the box's inner side walls. Take a chisel or knife or whatever and scrape that stuff off, and get rid of any other stuff you might find in there while you're at it (for structural stability purposes, a lot of speakers have little wooden blocks glued into their corners on the inside).
If any chunks of wood have come out with the glue globs, or if you had a missed cut or two when you were slicing out your door, fill in those errors with either some wood filler or some epoxy putty (I recommend J&B Waterweld, as it's the best stuff on the market). After that, sand the inside of your box until you're pleased with its overall smoothness. For the record, I was chasing after Billy Dee Williams level smoothness, so I had to sand mine for a LONG time.
Step 5: Paint 'n stuff
Once everything structural is in place, lay on a couple coats of paint (unless you want to showcase the gorgeous particle board on the inside of your cabinet). I won't tell you how to properly administer paint, as anyone who doesn't know how to do that almost certainly wouldn't know how to read either.
Step 6: Hinges and Sexiness
Once that's done, you're free to stand your new kickass media cabinet up and see how it works. If you did everything correctly, your new media stand should be the envy of every Tom, Dick, and Harry within a 10 mile radius of your living room (though I won't be envious at all, as I already have my own speaker media cabinet... also, my name is not Tom, Dick, or Harry).
I'll hopefully be porting a lot of these projects of mine over to instructables in the coming weeks, but not all of them, so if you want to see some of my other stuff please check out: My Drivelicious Blog