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The most obvious hardware difference is the physical camera button found on the Lumia 800. The volume and power buttons are the same and are actually the only part of the hardware that is a bit weak with some wobble to them.
The speaker is found on the bottom of both devices as you can see by the grille here.
The other obvious physical difference is the screen size and capacitive buttons. The Lumia 800 display is 3.7 inches due to the inclusion of the three capacitive touch buttons while the N9 is 3.9 inches.