What this means for learners
Yao and Karen share 24 sounds — roughly 49% of Karen'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 15 sounds found only in Karen 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 25 sounds not used in Karen. Native Karen 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.