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Not sure if this answers your question (or just tells you something that you already know) but...
The water jacket on a water-cooled engine does attenuate (quieten) the sound considerably. Not only does it quieten the motor overall, but it affects different frequencies to a greater or lesser degree. So in the first instance, that's one of the key factors for a unique "air-cooled" sound
Installation (think of the engine bay as an echo chamber) and the relative noise of the exhaust and other ancilliaries also affect perceptions (I think of the 500cc F3s, where the exhaust noise is so loud it drows most of the air-cooled sound).
I've not heard of resonating fins, but it sounds reasonable.
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