The word is pronounced kil’ayim and not kilayim, as there is a sheva under the ל. People sometimes mispronounce it as kilaim, as if there is a hirik under the א of כלאים. This reminds me of another common mistake. If you google you will find that many refer to the concept of “shomer pesaim (petaim).” Yet this is a mistake. The verse in Psalms 116:6 reads: שֹׁמֵר פְּתָאיִם. The second word is pronounced pesayim (petayim), as the א is silent. Another example where the א is silent and many people make a mistake is with the name דניאל. Even people who have this name often pronounce it in Hebrew as Doniel (or Doniellah for women). Yet the name is properly pronounced Doniyel: דָּנִיֵּאל. This is unlike the name אריאל where the tzere is under the א and the word is pronounced Ariel.
Marc B. Shapiro, “Which Direction is North?, Geographical Mistakes, and our Woke Universities”, The Seforim Blog (12 August 2021) [https://seforimblog.com/2021/08/which-direction-is-north-geographical-mistakes-and-our-woke-universities/], n. 38.