I'm inclined to think that plumminess is a state of mind, rather than a question of phonetics. Moreover, it's important to acquire an engaging plumminess, rather than that faintly off-putting tone that you hear from people like – well, I won't name names; but you've heard them on the news, I expect, talking about the local elections. For engaging plumminess, as I say, the state of mind is the most important thing.
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