What this means for learners
Sundanese and Balinese share 21 sounds — roughly 81% of Balinese's inventory overlaps with Sundanese. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 3 sounds found only in Balinese represent the greatest pronunciation challenge for Sundanese 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, Sundanese has 5 sounds not used in Balinese. Native Balinese speakers learning Sundanese 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.