"The problem is that many (possibly most) adults have lost the ability to recognize and learn "new" sounds that are not part of the sound system of their native language or dialect."
False. Anyone can learn new sounds and suppress an accent. It's difficult but not impossible, and adults manage it all the time, if they are sufficiently motivated (most are not, since accents don't usually interfere with comprehension).
"In many cases, someone could even use a course like this and THINK he/she is producing the native-speaker sounds accurately, ... tell the difference (or cannot realize that the difference is meaningful) between the native speaker's sounds and his/her own sounds."
No, I don't think so. Proper instruction along these lines does indeed eliminate foreign accents.
"I'm still reminded of a graduate student from South America (whose first language was Spanish), whom I knew 20-some-odd years ... obscenity. (!) I really doubt this particular person would have benefited from the "Lose Your Accent in 28 Days" program."
Some individuals are unable or unwilling to change, for reasons specific to themselves. That doesn't mean that it is generally impossible. Many people do successfully suppress their accents.
"On the other hand, I imagine that a strongly motivated student with a good ear, a flexible brain, and great attention to detail might possibly do very well with a self-study course like this."
Indeed, he might entirely eliminate his accent, which is the goal. So where's the problem? It's not the course's fault if some students are unwilling or unable to benefit from it.
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