regional.ar.zheismo
Zheísmo & sheísmo: ll and y pronounced 'sh'
The signature Rioplatense sound: ll and y are said as voiced [ʒ] (zheísmo) or, increasingly, voiceless [ʃ] (sheísmo) — calle ≈ 'cashe', yo ≈ 'sho'.
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Explanation
Where most of Latin America pronounces ll and y as a soft [ʝ], the Río de la Plata turns them into a strong postalveolar fricative. The traditional zheísmo is voiced [ʒ] (like the 's' in 'measure'), so calle sounds like 'cazhe' and lluvia like 'zhuvia'.
Since the mid-20th century the voiceless [ʃ] (English 'sh') has spread, especially among younger speakers in Buenos Aires — sheísmo — so yo becomes 'sho', calle 'cashe', and ya llegué 'sha shegué'. Both variants are heard side by side.
This is the single most identifiable trait of the Argentine and Uruguayan accent. It applies only to ll and y before a vowel; a word-final y stays a plain vowel (hoy, muy, soy), where the sound does not occur.
Examples
My name is Guillermo. (said 'sho me shamo')
Region: AR
It rained all night in the street.
Region: AR
Did you already get here? (pronounced 'sha shegaste')
Region: AR