What this means for learners
Dioula and Yorouba share 30 sounds — roughly 77% of Yorouba's inventory overlaps with Dioula. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 2 sounds found only in Yorouba represent the greatest pronunciation challenge for Dioula 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, Dioula has 9 sounds not used in Yorouba. Native Yorouba speakers learning Dioula 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.