This isn't likely to shock you, 12V is pretty harmless in that respect.

However if you do touch both ends of the wire to something metal, the entire speaker wire is likely to melt very quickly, and burn your hand. Melted speaker insulation may stick and cause further burns.

That said, it's a pretty cool trick to experiment with. Protect yourself though. Wear gloves and safety glasses. DON'T use jumper cables, or any heavy wires. Thicker wires can carry much more current before failing, possibly enough to destroy your battery in a spectacular, and dangerous fashion.

Try this with the wood still on the pencil. *edit* Try it outdoors of course.