Today I noticed that that a Skeleton Champion summoned his minion, and that the spell was cast from its mouth. Is that normal? If so, could it've been Thu'um? (Ayleids loved Magical knowledge and power, so maybe they were interested in the Thu'um too?)

I just ran a quick test. I added a Skeleton Champion to ToddTest in the CS, went into GodMode, COC'd to ToddTest and watched the Skeleton Champion fight a guard. What I saw was this: when the Skeleton Champion summoned his skeletons he seemed to raise his head and fling his arms outward in a gesture that could possibly be interpreted as shouting. But I didn't hear anything come out of his mouth.

My interpretation: this is just his "casting animation." But, I could be wrong.

It all depends on your interpretation. Kinda like the Nasca Lines in Peru. Nobody really knows.

Good find though, ashan.