I skimmed the links and don't disagree with what they seem to be asserting. It was clear in my own learning of foreign languages that I was missing, or maybe dismissing, many sounds at first, and that has to be pointed out to you by a vigilant teacher. It takes exposure to many different native speakers to pick up on what sounds are actually part of the language and which ones are idiosyncracies of a given individual. The more remote the foreign language is from your own the more that will be true. What I disagree with is that it can't be overcome, that it's universally impossible to become fluent in a foreign language as an adult due to the accent problem. I think it's analogous to learning to paint at a later date in life: some people will never get the hang of it and others will become "fluent".