[LazyH]

If you get a pair of drivers, even a couple low-fi ones, and follow a few basic guide lines on making the headphones it'll be hard to make headphones that don't sound good for the price. Open headphones are typically the easiest for the DIYer to make without compromising sound quality. I don't think I'm using quite the proper terminology but the acoustics don't care about your vocabulary, either open baffle (like a cd with a driver in that center hole) or open can (typically 1-2 inch pipe with the driver in one end, grado-ish design) are the easiest to make without messing up the sound. Avoid things that distort sound, and you won't have any problem getting good sound, if you can use a wood lathe to make two identical pieces, that would probably be the most reliable method.

 

[Angelo Inferno]

I wouldn't bother, my brother in law gave me some Sony DJ headphones with 50mmdrivers that had snapped at the headband.
Long story short, I get Peltor's free from work so the drivers found their way into a pair....... Crap........ Total and utter crap! Where the hell did the bass go? Even worse why does the treble sound so muddy?

Fun project, but i suspect your not going to end up with something to seriously listen to music with.

Put it this way, I got my sister some Sennheiser HD201's as stocking fillers last crimbo and for all of about £18 they pi$$ all over the Frankenphone Peltors.

That depends on how you put the drivers in there, did you add a baffle to mount the drivers in or did you just slide them in there? if you get drivers from closed backed headphones, they'll usually be less bassy because the closed back extends bass response, if you want more bass and it's not due to errors in mounting I'd pull a pair of drivers from open backed headphones since the drivers are most likely tuned to put in more bass than closed back to compensate for the lack of extension. But both of those statements, lack of bass and muddy highs, point towards improper mounting with echo and bass leak from behind the driver.