Since in the game he speaks Akkadian not Sumerian, literally anyone capable of speaking Sumerian would be better at speaking Sumerian. It's true that most of what we know about Sumerian is viewed through the lens of Akkadian, but that doesn't mean we know nothing about it. We have a reasonably sized corpus and as fair an understanding of its phonology as one could wish, under the circumstances. They could have hired a scholar versed in Sumerian about as easily as one versed in Akkadian, and generally speaking they'll be the same people--I doubt there are any Sumerologists who aren't also Assyriologists due to how much larger the Akkadian corpus is. Speaking Assyrian Neo-Aramaic might help understand the grammar a little (maybe you've heard the joke: the easiest Semitic language to learn is the third one), but it won't help with pronunciation as Neo-Aramaic has pharyngealized emphatics and a generally very different phonology from Akkadian. Classical Arabic might actually be grammatically closer, but the phonology wouldn't be any closer. As far as being able to pronounce Akkadian, hiring a Georgian or other Caucasian might be the best bet. In the end, if you want someone who can pronounce a dead language, you want a linguist or you want an actor with a language coach. Gilgamesh's actor butchers the pronunciation and pronounces a Sumerogram literally (not sure how familiar with Akkadian you are, but Sumerograms are logograms that stand for a word in Sumerian but meant to be read in Akkadian--so when Gilgamesh reads as "ki engi" he should read it as "Šumeru"). Sweden and the US actually have higher Assyrian populations than the Middle East due to heavy emigration from their homeland, but like I said no variety of Neo-Aramaic is particularly similar to Akkadian.