What can be observable in the process of acquisition of tones by L1 speakers of English? Chinese (and a host of other languages in South East Asia) already gives us an enormous database of observable patterns. African languages (in this case Yoruba) occupy another level of the problematic realm for those merely accustomed to a language based on intonation, stress and inflections. Why is it funny when my friends call me in a way that rhymes my name with Cola or “caller” rather than with the uptalk mode of pronouncing the “sugar” in “sugar daddy” or the “brother” in “brotherly love”. Tone is music, rising and falling as needed. What makes it imperative that speakers of English relate to it only in one direction, viz (usually) as a high-low in a two syllable word? Why will “Bolaji” sound like “allergy” rather than the “beautiful” in “beautiful girl”?
What other nuggets are observable? How much proficiency can an L1 English speaker really acquire in a tonal language like Yoruba? With the many years of study by people like Karin Barber and (perhaps) Susanne Wenger, could they/did they pass the native-like proficiency test? What is the bar for native-like proficiency anyway?; and besides the general list of impediments across second language learning processes, what are the specifics in L2 tonal language learning that presents the greatest obstacles? And how does it happen? It is after all equally easy, equally difficult to learn any language either at L1 or L2 level given an equal and sustained level of interest and low affective filter. Jargons, jargons.
A linguist might know, or at least be neck deep in the long process of finding out.