[thaumasiotes]

> if an adult is set in their ways and knows a particular set of phonemes, (or even tones, etc.) is it more difficult than a blank-slate child who has no prejudice about hearing and learning new sounds?

The answer is sort of "yes". If an adult is set in their ways and knows a particular set of phonemes, they will have a more difficult time with the phonemes of a new language than an infant would.

However, "learning new sounds" is not a correct way to think about it. You're born knowing all the sounds. You unlearn the differences between certain ones. If you, as an adult, have unlearned a difference that matters in your target language (because it didn't matter in your native language), you will have trouble with that difference. An infant can't have this problem.

Note that the cutoff point where an immersed child will fail to learn the pronunciation of a new language "automatically" is somewhere in the late teens, though.