We don’t really, not for certain; we can make a very educated guess though based on reconstruction. Sumerian in particular is tricky because not only has it been extinct for a long time, but there are no languages that are directly descended from it (like Italian is descended from latin for example). It did have a lot of influence on Akkadian, which is a semitic language, so a lot of the reconstructed pronunciation is based on what we know of Akkadian based on patterns in the semitic languages and comparing Akkadian cuneiform texts to Sumerian cuneiform texts.
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