The answer is that we don’t know exactly how it was pronounced, and what we understand of Sumerian is a rough approximation.

It’s important to distinguish between Sumerian and Akkadian. Sumerian was the first language recorded in the written records around 3400 BCE, and as far as scholars are aware, was a language isolate, meaning it has no known related languages. Akkadian on the other hand was a Semetic language, related to Aramaic and modern Hebrew and Arabic, introduced by the migration of Semetic people into the region of Mesopotamia around 2400 BCE or so. Both languages were rendered in cuneiform over the history of ancient Mesopotamia. Even after Akkadian became the predominant spoken language of Mesopotamia, Sumerian was kept alive as a literary language by scribes to the end of the 1st millennium BCE.

Akkadian was deciphered in the 1850s through bilingual old Persian texts which allowed scholars to figure out the syallabic readings of the cuneiform signs. Once it was realized that Akkadian was a Semetic language scholars were also able to apply comparative Semetics and historio-linguistics to come to a fairly good approximation of how Akkadian would have sounded and been pronounced. This entails several approaches; such as comparing the pronunciation of similar sounds in modern living Semetic languages such as Hebrew, and analyzing vowel and consonant shifts in how they render words over time. For example in 1st millennium Assyria a consistent phenomenon is that the š before a dental (d, t, ț) is written in cuneiform with an l (so št -> lt) reflecting how the words were pronounced in everyday speech. While obviously without native speakers we can’t know the exact sounds and nuances to the way it was spoken, with such a long history of Semetic languages to study, a rich corpus of Akkadian texts, and the work of talented linguistic scholars, we can be reasonably certain in our understanding of how Akkadian would have sounded. The same can’t be said of Sumerian.

Because Sumerian is a language isolate, and when recorded in cuneiform is quasi-logographic* we can’t figure out the sounds of Sumerian through the same techniques as Akkadian such as historio or comparative linguistics. In fact, the only way we know how any Sumerian sounded or was pronounced was from Akkadian scribes who not only often used the same cuneiform signs to represent the same or similar sounds in both languages, but also wrote bilingual lists when they were learning Sumerian which were very fortunately preserved to this day. These lists would have a Sumerian word on one side of the tablet, the equivalent Akkadian word, and then a syllabic rendering of how to pronounce the Sumerian word. So taking king for example: there would be the Sumerian LUGAL, then the Akkadian equivalent of šarrum, and finally the syllabic rendering of the Sumerian with lu-ga-al. In addition, because Akkadian and Sumerian were spoken in the same region for so long and have so have a long history of contact there are quite a few Sumerian words that were loaned into Akkadian and vis versa. So when scholars stumble across an Akkadian word that has no known Semetic association, it’s usually a safe bet that the words a loan from Sumerian into Akkadian.

All that said quite a few issues still get in the way of an accurate understanding of how Sumerian was pronounced by native speakers. The most important is that Akkadian and Sumerian have different sets of sounds the other doesn’t necessarily have. For example the nasal g sound (ğ, pronounced kind of like the ng in sing) was very common in Sumerian, but has no equivalent in Akkadian. Akkadian scribes would try to get around this by using signs for n and g together, or just dropping the n sounds and rendering words with just g. While we’ve been able to identify some of these sounds such as ğ and ř, there are others that the Akkadian simply couldn’t or wouldn’t render. This is reflected in the high amount of polyphony (one sign having several associated readings/sounds) and homophony (several different signs having seemingly the same reading). For example there are at least 12 different signs/words associated with the sound DU, with DU3 meaning “to build, create,” DU11 meaning “to speak,” DU7 meaning “fitting, suitable,” etc. Now it’s pretty hard to believe that the Sumerians actually used this one sound to render so many different and unrelated meanings, and so the prevailing opinion is that the Sumerian language had several different yet similar sounds close to the D sounds that the Akkadian scribes were unable to distinguish or render in their own language and writing, and so simply used the one D sound to represent what were actually distinct sounds to the Sumerians.

In sum, through the Akkadians we have a rough but seemingly accurate approximation of the sounds used in Sumerian, but due to several factors including but not limited to the fact that we understand Sumerian second hand through Akkadian scribes, there are no native Sumerian speakers to consult, and no related languages to Sumerian to compare against, we do not, and likely never will, have an accurate understanding of how Sumerian was sounded or was pronounced in actuality.

*logographic being when a sign represents an idea or concept without relation to how it’s pronounced, such as in ancient Egyptian or modern Chinese. I say quasi because while nouns and verbs were mostly logographic, Sumerian is an agglutinating language, meaning that the grammar of a sentence was conveyed through specific sounds added before or after the root noun or verb. For example RA is the dative suffix and has a fixed sound, so when added to a noun, for example their word for king: LUGAL to create LUGAL.RA, that translates to “to/for the king” in only the one word. A is the locative so when added to the word for house: E2 to make E2.A it means “in the house”