Appear to be in A flat major. Note the melody (soprano) line basically stays put while the other parts drop out from under it forming an A flat chord, an F minor, D flat, E flat, then resolves back to A flat chord on “sunt”.
In contrast, check out these other links which also purport to be Michael Haydn’s Tenebrae Factae Sunt:
This other, entirely different piece, appears to be in E flat major, with a rising soprano melody line starting on the 5th, rising to the 7th on “brae”, then stair-stepping up tones to the high 3rd above tonic on “sunt”.
It’s not entirely unheard-of for one composer to write two different versions of something, although it’s rather rare. WHEN they do, one would normally be able to successfully Google something like “Michael Haydn” + “tenebrae” + “two versions” and find a descriptive page. Nada. (At least nothing usefui or truly topical).
I suppose there could be two different Michael Haydns. But each of the two versions of the song is attributed (sometimes) to Michael Haydn and (sometimes) to Johann Michael Haydn. I did see one ChoralWiki page somewhere in passing that said a version (dont’ know which) of Tenebrae was misattributed and was actually written by the more famous Franz Joseph Haydn.
I supposed for a fairly long time that the small number of references to the first composition (the A flat version) were simply wrong and that sooner or later I’d find a youtube page giving the correct attribution, but after I found a 3rd source saying Michael Haydn had written it (and none saying someone else had done so) I decided that wasn’t too likely.