Computer speakers are usually shielded, but with the rise in use of flat panel screens, there are some that are not now.
If you just rest the drivers removed from the plastic box on the grill, it will sound awefull and not go loud before the speakers are distorting. You need to make the speaker sealed to the surface its poking thru, and ideally the volume inside the box should be the same as what it came out of. If they are cheap speakers they wont have being designed, just assembled so chances are what they come in will be wrong anyway.