Something excellent or desirable is plummy, like the plummy seats you scored for a popular Broadway play — right up front where you hear every word of the actors’ plummy, or rich sounding, voices.
The adjective plummy is, quite straightforwardly, a way to talk about anything that tastes like a plum. Imagine someone sipping from a glass of wine, describing its plummy notes. In Britain, it's used far more often as an informal way to say "very desirable," like an actor's plummy role in a new movie, or "upper class," like your wealthy cousin's plummy accent. It's also applied to sounds that are rich and full.