What this means for learners
Yao and Yay share 27 sounds — roughly 55% of Yay's inventory overlaps with Yao. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 11 sounds found only in Yay represent the greatest pronunciation challenge for Yao speakers. The adult brain tends to map unfamiliar sounds onto the closest native equivalent — a process that produces the characteristic "accent" of a second-language speaker. Learning to hear and produce these sounds as distinct requires focused ear training, not just repetition.
Conversely, Yao has 22 sounds not used in Yay. Native Yay speakers learning Yao will face the mirror-image challenge with these sounds.
Phoneme inventories from PHOIBLE. Data reflects one documented inventory per language; some variation exists across dialects and sources.