Minor point about banjos: Saying they "have the sound-hole at the back" is missing quite a lot. Decent modern banjos usually are not open in the back. They have a resonator, a kind of rounded wooden shell covering the back. There are sound holes all around the outside rim of the resonator, facing forwards. But the main thing about a banjo, with regards to its sonic archictecture, is that the bridge is held tight against a drum head by the strings. The vibration of the strings is transmitted directly to the head of a drum. This is quite a lot different than just having a sound hole, and is what gives the banjo its characteristic sound, a rapid attack and very very swift decay with very little sustain for a plucked stringed instrument.

The other thing I've wondered is just exactly what sound holes are for. The name "sound hole" I believe is misleading. It's not necessary to have a hole to "let the sound out" as it were. After all, the bridge transmits the vibrations of the strings to the thin wood surface of the violin body or the guitar top or a banjo's drum head, or whatever. The point is, the vibrations are transmitted to some kind of "tympanic membrane" that can move some air. I think the "soundholes" are more along the lines of the ports in bass reflex speakers. Well, maybe in simple terms, the ports in bass reflex speakers are there to "let the sound out." But in this last paragraph, I'm just speculating.