The Rigveda is the the earliest unbroken kept record.

The Sumerian tablets are likely older, but they were forgotten and then re-deciphered. The transliterations still carry educated choices made by a scholar (there's only so much you can do when the record breaks for a long time).

Meanwhile, we know exactly how people spoke those early vedas because there is a tradition that keeps chanting it, without stopping, since before it was written down. That chanting has error correction mechanisms, redundancy and other impressive features that further place the Rigveda in a unique place among earlier works.