All you need is:
A ball chain
A 1/8 Inch Female (3.5mm) to 1/4 Inch Male(6.35 mm) Audio Adaptor
A 4mm drill bit
A drawing pin or pointed knife
WARNING: depending on which brand of adaptor you have, you may have to alter the steps to suit it, so that the adaptor is still usable when drilled. Some adaptors have completely metal insides, and without a metal drill bit, this instructable won't work.
Step 1: Press drawing pin/knife into either side of the adaptor
Step 2: Drill
Try to avoid the metal areas. This may take some trial and error.