What sound features make the Souletin dialect different from many other Basque dialects?

The Souletin dialect of the Basque language is distinguished from other dialects by several unique phonological features acquired through internal development or language contact.

Unlike the standard five-vowel system found in most Basque dialects, Souletin has developed a sixth oral vowel, which is a rounded e or i. Additionally, the dialect includes the following sound characteristics:

• Nasal vowels: Sounds produced with airflow through the nose.
• Voiced sibilants: A set of sibilant sounds that are voiced, contrasting with the voiceless sibilants common in other varieties.
• Voiceless aspirated stops: Stop consonants accompanied by a small puff of air.

Furthermore, while the aspirated h was once common across the language, it has largely disappeared in many areas and is now considered a feature peculiar to the northern dialects, including Souletin, where it is retained at the beginning of words and between vowels.1