What this means for learners
Ilocano and Sundanese share 18 sounds — roughly 69% of Sundanese's inventory overlaps with Ilocano. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 8 sounds found only in Sundanese represent the greatest pronunciation challenge for Ilocano 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, Ilocano has 7 sounds not used in Sundanese. Native Sundanese speakers learning Ilocano 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.