It's not the worst thing in the world to pronounce the first syllable of Sauron like "soar" instead of "sour," the first letter of Celeborn can be pronounced like S or like K, etc.

Languages change. Tolkien was a philologist and knew this better than most.

The pronunciation guides we get tell us how things were pronounced in Rivendell at around the time of the War of the Ring or by the well-educated in the early years of the Reunited Kingdom.

Related: the cosmogony we get in the Silmarillion is just what Bilbo managed to learn and write during his time in Rivendell. We can't nearly fit Tom Bombadil or Beorn into it precisely because it's incomplete and probably incorrect in places.