Without a microphone it will sound like any home movie. You'll here what's happening, but it will be in no way releasable to any sort of audience. With a microphone attached it might sound almost passable, or it might be full of static, interference and hiss. You'll have no idea though, because you can't monitor via headphones. You'll just find out once you get home and review the footage. Maybe it sounded okay, or maybe it's completely unusable and you wasted a day of shooting.
Please also understand buying a DR-100 and a decent microphone isn't going to give you "Inception style high def" sound. It will just get you sound that is usable at the indie level, providing the equipment is operated correctly. Hollywood level production sound kits can run into the tens of thousands of dollars, if not hundreds.