Electrical – Testing cabinet screw heads for electrical current

Since you're painting anyway, it doesn't make much sense to use the more expensive hardwoods like quarter-sawn white oak or blistered maple - quite ordinary poplar should do very nicely unless you plan to put a huge amount of weight on those shelves. Poplar is strong and dense, and it paints exceptionally well.

Note that you'll need to edge-laminate two or more planks to get your full 15" depth if you use poplar.

Another entirely viable option would be to use 3/4" MDF. It comes in sheets (you can often buy half-sheets and quarter-sheets). It's very heavy (about 96 pounds per full sheet). It's also very rigid and dense and takes paint very well. So well, in fact, that it's sometimes used as an inexpensive siding on condos. It doesn't tolerate moisture, though, so be sure to fully paint all surfaces.

Yes! For goodness sake, do NOT put any commonly available potential (ie 120V) on the lines, they make very easy to use tools for this. Most sources in your house will be voltage sources, vice current sources, so how much current results is a factor of the resistance in the line. Because these speaker wires will have low resistance when you find both ends, you will get a LARGE current in the lines which could destroy your fuse/breaker or even cause a fire.

What you are describing/seeking, though, is essentially a resistance measurement (ohm reading). Get a multi-meter, the "ohm"/resistance measurement ("upper-case" omega) sends a small current down the wire and measures the voltage, to determine the resistance (using the equation v=ir). In your case, you know you will have found the other end when the measurement of resistance goes to near zero (after starting at off scale, or pegged). I would wrap one of the speaker ends around the black lead (color chosen for no particular reason) then go to each wire on the other end until you get the near 0 reading of resistance.

If the wires are too far apart, make an extender out of an available piece of speaker wire. Simply twist the free piece of wire to one end of the speaker wire on the wall to extend it far enough that your red and black meter leads are long enough to reach both ends. Then continue with the resistance measuring technique described above. Now, use the red lead of the multi-meter to go through each wire until you find the right one. Mark those wires and move on to the next.