Since i made these i found out they have already been thoughtof :( as has the speaers-in-a-hat i also made a while back (though mine had an mp3 player builtin ;)) See: http://www.hearmuff.com/
This is my first instructable, and i'm making these for my girlfriends birthday.. go nerd presents!
Step 1: Tools and parts
A pair of ear muffs
A set of headphones (supra-aural or circum-aural)
A set of earphones, for lead replacement (optional)
A soldering iron (or gun)
Glue (plastic glue or superglue - not shown)
And possible a kinfe/scissors/clippers.
Step 2: Mounting the drivers
Remember to get them pointing inwards! Then add a little glue to ensure they stay where theyre supposed to. i used plastic model glue (Airfix type plastic cement) but superglue would probably work just as well if not better.
Leave this to dry, then proceed.