> but it's impossible to confidently reproduce the sounds unless they are already completely distinct in our heads/ears
interestingly, i think this isn't always true -- i was able to coach my native-spanish-speaking wife to correctly pronounce "v" vs "b" (both are just "b" in spanish, or at least her dialect) before she could hear the difference; later on she was developed the ability to hear it.
I had a similar experience learning Mandarin as a native English speaker in my late 30s. I learned to pronounce the ü sound (which doesn't exist in English) by getting feedback and instruction from a teacher about what mouth shape to use. And then I just memorized which words used it. It was maybe a year later before I started to be able to actually hear it as a distinct sound rather than perceiving it as some other vowel.
interestingly, i think this isn't always true -- i was able to coach my native-spanish-speaking wife to correctly pronounce "v" vs "b" (both are just "b" in spanish, or at least her dialect) before she could hear the difference; later on she was developed the ability to hear it.