Lamport says that either pronunciation is acceptable, and back when he gave talks about LaTeX, he would carefully switch back and forth between "ay" and "ah" each time he uttered "LaTeX". Or so he claims.
> Lamport says that either pronunciation is acceptable […].
This fits what Knuth said about the issue in 2006: “[…] lah-tek, lay-tek, the author never has decided how to pronounce it […].” [1]
I've always heard (since the 80s) lah-tek. But I've never asked Lamport whose pronunciation is presumably canonical
> TeX's creator Donald Knuth promotes a pronunciation of /tɛx/ (tekh) (that is, with a voiceless velar fricative as in Modern Greek, similar to the ch in loch).
So, LAH-tekh.
Although there are a number of OK ways, lay-tech seems to be what most knowledgeable people in the community use.