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Different parts of Eastern Europe had different dialects.
For example, Ukrainian, Polish and Hungarian Jews pronounce a "Shuruk" as a EE, or a Komatz as an oo (so Boruch becomes Burich).
From where did this pronunciation come? Most Ashkenazim have a (somewhat) similar pronunciation to Sefardim (For example, Sefardim would say "Baruch" while Ashkenazim would say "Boruch"). Moreover, Poland had a closer connection to Lithuania (they were under a joint Monarchy before both being swallowed up by the Russian Empire) than to Ukraine.
Moreover, there are videos of the Lubavitcher Rebbe (who was from Dnepropetrovsk which is in the center of Ukraine), and he speaks with a "conventional" pronunciation.
Where and why did the different pronunciations start?