This combination of a resource pack and a command block invention adds Witch Sounds into the game.

You can add it to the world either by inserting the schematic using MCEdit or by pasting a command into a command block and pressing a button. Once you have the command block setup, press the button that is on one of the command block lines to start it up - then play and enjoy hearing their sinister laughter. You also need to add the resource pack to the game the usual way.

There are four sounds in the pack/invention:

• Footstep sounds
  
• Idle sounds
  
• Taking damage sounds
  
• Death sounds
  

All of them have several variations in the resource pack. If you want, you can switch out the resource pack or make one yourself - just add sounds for:

mob.witch.step
mob.witch.idle
mob.witch.hurt
mob.witch.death

If you want to paste the command, copy it from here, and the resource pack is available separately here.