Headphones come in several different styles. You have the big ones that completely encircle the ears, and can either seal out exterior noise or let it in, depending on the shell. There are smaller headphones which sit on top of the ears, and usually have foam pads that will not seal out exterior noise. You also have the small earbud types, which sit just in front of the opening to the ear canal, and will let in some exterior noise. Lastly, you have the plug type that fit inside the ear canal itself, and will seal out exterior noise (the last two probably wouldn't be a good choice for someone w/hearing aids). There are headphones made specifically for metal detectors and have their own built-in volume control, but you can use a set of regular music headphones as well.
Perhaps you can tell the readers what type or size you prefer -- it would help to narrow down the field quite a bit. Also, in case you have a detector that does not have a volume control, and you end up with a regular headphone that does not have it's own volume control, you can get an in-line volume control at Radio Shack for under ten bucks.
hth,
Phil
