Hi KuaiLe, I'm not sure about plum-voiced, it's not a term I've ever heard. My guess is that it is round and smooth, like a plum. But, perhaps, someone else will know for sure. To say that someone's idealism is "touching" is to say that it is "moving" in an emotional sense. To say that it is "profoundly naive" is to say that it is deeply naive or unworldly. You can find synonyms and definitions of "naive" in any dictionary.
I hope that helps.
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