What this means for learners
Iai and Toba-Batak share 23 sounds — roughly 44% of Toba-Batak's inventory overlaps with Iai. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 4 sounds found only in Toba-Batak represent the greatest pronunciation challenge for Iai 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, Iai has 29 sounds not used in Toba-Batak. Native Toba-Batak speakers learning Iai 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.