What this means for learners
SIONA and Sundanese share 15 sounds — roughly 50% of Sundanese's inventory overlaps with SIONA. 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 Sundanese represent the greatest pronunciation challenge for SIONA 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, SIONA has 15 sounds not used in Sundanese. Native Sundanese speakers learning SIONA 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.