Tutorials/Music disc farming

Music Discs help break the silence of the world, but in order to obtain them, you must be very lucky or skilled. They are dropped by Creepers who are killed from Skeletons. Not only is it hard to get a Skeleton and a Creeper together, but it's harder to get the Skeleton to shoot the Creeper.

But that's not the worst part. After being hit, the creeper will try to retaliate in the only way it can - by exploding, making you lose your chance to get a Music Disc. As such, discs would be efficiently obtained if:
 * 1) The Skeleton and Creeper stand still in a controlled environment.
 * 2) The Skeleton has a way to hit the Creeper, while trying to hit you.
 * 3) The Skeleton and Creeper must not get too close to each other, lest the latter explode.

Thus, the best way to farm for Music Discs is to create a "factory" - a structure organized for the purpose of generating discs, easily and quickly.

Controlled farm
A simple (but not easy) way to obtain discs without too much trouble is to build a mob system next to a skeleton spawner. Make the system end in a fall exactly 22 blocks above the ground and exposed to the sun. The fall will kill spiders and the sun will finish off the undead, leaving only the creepers. Then build a mob elevator for the skeletons to travel up and shoot the creepers. Remember to separate them with fences, and cover the skeleton spawner area.

General setup
An easy way to get both enemies stuck in place is to trap them in a 1x1x4 environment. 4 blocks of height are recommended to make sure the mobs can't jump out (or you can block mobs using extended pistons), and that the Skeleton has enough space to shoot through. In order for you to see the mobs in action, it's recommended that you build the structure out of Glass blocks. Build the whole thing underground, so you can just guide the mobs to their respective hole while on the ground, and take care of business underground.
 * Note: As of 1.2.3, mobs will detect tall drops and will avoid them.

You'll want to have them in separate chambers, but there has to be a path between them that allows the arrows to reach the Creeper's chamber... without letting the two mobs get too close to each other. Making a small tunnel is effective, but also limits the Skeleton's aiming window. For the best results, you should use Fences - the Creeper can't jump over them, and it leaves more space above for the Skeleton's arrow to shoot through.


 * Note: Before 1.0.0, fences occupied an entire block, but with 1.0.0, they've started taking less space, allowing the Skeleton and Creeper to come closer to each other. 3 fences were enough to keep them apart before 1.0.0, but now, you have to use 4 fences, unless you want the Creeper to blow up.

After you've created chambers for both of them, with fences in-between, it's time to make a small hole on the Creeper's side, so the Skeleton can see you, and try to fire. Of course, you also have to take into account that the Creeper will attempt to blow you up as well, so make sure the hole is 3 blocks away from its room.

Retrieving the discs
Now there's only one thing left. After spawning, the discs will remain in the Creeper's hole. Unless you want to get into the hole every time to retrieve it, you must make a small tunnel, adjacent to the "Skeleton → Creeper" line, that passes through the bottom of the Creeper's chamber, and make a water stream pass by. That way, any dropped discs will go through the water stream and come out of the structure. You can even make them travel to the place where you must stand to wait for the Skeleton shots.

...There is one small problem, though. Due to the fences' smaller hitboxes (read above), there is a chance the discs will drop on the fence's block, and not be taken by the water stream. One way to retrieve these is to dig under the floor and get them. Alternately, you could remove the block under the fence closest to the Creeper, and the disc will fall below, where you can guide it with water streams or do something else with it.

Trapping the mobs
The factory is ready. Now, you need a good way to guide the mobs to their holes. Assuming you did the factory underground, you can just grab the attention of a nearby Skeleton/Creeper and make them fall on the corresponding hole. There are 2 things you can do to make this process a lot easier.

Trapdoors
To make sure you don't accidentally send the wrong mob to the wrong hole (very dangerous if you send a Creeper to a hole with a Skeleton already inside), you should use trapdoors, and only open the respective door when you're luring an enemy, closing it when it gets in.

NOTE: Trapdoors won't work as of 1.2.3, The only known way to get the monsters into the desired hole (as of this moment) are using Pistons.

Glass platform
Make a large glass platform a few blocks above the ground, so you can guide the enemies more effectively from above. Add some ladders so they can't climb up after you. After luring one, quickly run to the entrances, open/close the right trapdoors (read above), and climb up the ladder. Then, just stand above the correct hole, and watch them fall right in, adjusting yourself as necessary. Remember to create small walls on this platform, or you might fall off when you're not looking.

Finishing up
After all that's done, you just need a way to get underground. Make a path where you can travel up and down by ladders, preferably next to the waiting spot. The waiting spot is next to the small window on the Creeper's side, so the Skeleton can see you, shoot, and hit the Creeper. The Creeper is too far away to retaliate, so it can only stand there and take the shots. Around half of the Skeleton's arrows will hit the walls instead of the Creeper, due to the shots' random nature, but it'll still kill the Creeper in no time. You can also help and land some attacks yourself, just make sure the Skeleton gets the finishing blow.

You can also cram a lot of Creepers in the same hole; they'll all be killed eventually. You can then do something else while you let the Skeleton do all the work, seeing as you don't need to move from your spot. If you channel the water stream right, you can get the discs to travel all the way to the place where you need to stand, removing the need to go grab the discs manually every once in a while.

Just make sure you pay attention to the sound. After all Creepers are dead, you will stop hearing splashes, and, if you're not fast enough, you will take damage from the Skeleton.

Extra details
These things aren't necessary, but might make the factory work more efficiently.

Protecting the Skeleton from the sun
Suppose you gather up a lot of Creepers during the night, and decided to start the onslaught. You get into position, only to notice the Skeleton is burning up due to the sunlight. A simple solution is to place a solid block above the Skeleton's hole, like on the glass platform, or better yet, above it.

Coloring the entrances
Decorate the entrances to each hole with Green and Light Gray wool. That way, when you lure a mob, you'll be able to quickly tell where to take it to.

Window to punch the Skeleton
On one of the sides of the factory, make a small hole on the Skeleton's wall so you can punch it. This is useful if the Skeleton somehow got its sights locked onto something else. In this case, you can just hit the Skeleton to get its attention back. In Single Player, you can create a window by its feet and hit it without fear of being hit back. This doesn't work in multiplayer, so you need to make a window by its head, and accept the almost-certain counter-attack.

Lowering your waiting spot
There doesn't seem to be much of a difference, but if you want, you can place a slab on the ground in the place where you're meant to stand, so the Skeleton will aim lower. If you feel this is giving better results, keep the slab, otherwise flatten the floor again.

Scheme
Here's a scheme for an effective music disc factory.

Side view
Side view where the water doesn't lead to the player's spot.

Question marks indicate the positions of the Skeleton, Creeper (with water on its feet), and player, respectively.

Top view
Underground scheme only, with the water leading to the player's spot.

Question marks indicate the positions of the Skeleton, Creeper (with water on its feet), and player, respectively.

Video Tutorials

 * Video tutorial with Commentary
 * Video tutorial on YouTube