nope, they have different sizes.

you have true 7.1 surround headphones the ones with multiple drivers? I think you need to use those kind of cans so that you could use your 7.1 option.

7.1 headphones use multiple 3.5 jacks the plug to the 5 recievers behind your pc. You can buy cheap soundcards like what purple is suggesting but don't expect much. Besides, 20 bucks is cheap it woldn't hurt to try.

Yes go contact the seller and send it back, or something happened to the diaphragm because of the shipping then you should ask thos headphone repair guys for professional analysis.

Your onboard audio is above average, given that you can use full sized 7.1 headphones in it, buying a soundcard between $0-$70 wont significantly increase the sq by 15%-25%, you won't notice the change much, unless you buy high end sound cards. But if you really want some improvement, I...