I'm intrigued by sounds that we can't make.
Sounds that a 2 year old routinely say are impossible for adult (or even later child) learners to master. The most obvious example are the click consonants found in some southern African languages, such as Xhosa, which I'm told are virtually impossible to master after infancy. Here's an example of such:
But there are other, less egregious examples that are nonetheless still hard for language learners. I was listening to a video this morning. A Dutch speaker was speaking in English pronounced "with" as "wis." The English "th" sound is one of those difficult for native Dutch speakers to master. (I'm reminded of a book I read in high school in which a native Dutch speaker, attempting to learn English spoke of having to exaggeratedly say, for example, "thee-ank you" to try to get the sound out).
But our friends in Amsterdam aren't the first, of course. Judges 12.5-6 recounts the first recorded such problem:
"And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right."