Is it possible by doing simple things to remove the speakers from, say, a clockradio, or a radio and attach a 3.5mm out so you can put your headphones in it, instaed of using the speakers?
Ya, you can do that, just make sure the radio can go to a low enough volume. Or use headphones with a volume control
Not really, they just seem louder because they are closer to your ear. Sound volume drops off exponentialy depending on how far away you are. So 75db, at 1 inch is MUCH louder then 75db at 10 feet.
volume drops off by the inverse square, and then some, because of the resistance (negligible at shorter ranges, for the msot part) you could jsut stuck a 100ohm resistor in series and see how it works.
What about the opposite of doing that, so hooking up a box to let's say an MP3 player headphone output? Would that work (without getting your batteries to 0V within an hour)?
You need some amplified speakers. Often you can find some that take batteries, they are made just for connecting to a protable radio. If you are in the US RadioShack carries them.