Earlier quoted context omitted.
I thought it was "Lah tech" too: It's named for its creator, Leslie Lamport. "Lay tech" wouldn't fit that.
The creator says you can pronounce it any of the canonical ways; he's good with them all. One of the hardest things about LaTeX is deciding how to pronounce it.This is also one of the few things I'm not going to tell you about LaTeX, since pronunciation is best determined by usage, not fiat. TeX is usually pronounced teck, making lah-teck, and lay-teck the logical choices; but language is not always logical, so lay-t…
So, LaTeX is pronounced "Lay-tech" with the ch from "Bach".
"Lay-teck" is also correct, but "Lay-tecks" is completely wrong.