In Dune, Ix is the universe's Japan, creating gadgets of varying utility.
How do people in Herbert's universe pronounce the name? Is it "Nine" and the inhabitants are "Nine-ians"? Or is it "Icks" inhabited by "Icksians"?
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It's pronounced as written. Ixian = "ick-see-ann"
You can hear a clip of Frank Herbert reading from God Emperor of Dune to get a pronunciation guide
In-universe, the inhabitants of Ix ("icks") largely aren't aware that the name of their planet derives from a small joke about the position of their planet in their solar system
... They are surface truth. As well ignore what lies beneath that surface as … as try to understand my birthplanet, Ix, without exploring how we derived our name from the fact that we are the ninth planet of our sun. No … no. It is not enough to see Dune as a place of savage storms. It is not enough to talk about the threat posed by the gigantic sandworms.
To add another source to Richard's excellent answer.
The Lynch movie Dune (1984), produced under Frank Herberts guidance, also establishes the pronounciation of "icks" for IX when a Guild navigator has a nice talk with the Padishah Emperor Shaddam IV. See here (youtube link).