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