Is this wrong?

'Hay' is a mass noun, surely, like 'milk' and 'mud'. Many of the lyrics of carols sound slightly ridiculous if you think about them.

Hay is a sort of plural noun, surely? You certainly don't get one hay.

I do agree that 'among' sounds odd and that 'amidst' would be better.

I think amongst sounds better than among. Amidst sounds nice as well.

I agree, MontanaGal. All the other suggestions make it seem (to me) as though Jesus was somehow mixed up with the hay!

I think "upon" the hay sounds better.

Still think among needs plural noun

It couldn't be "on" *trisher" because that would only be one syllable and wouldn't fit the music? Not that I have a clue about phrasing.

"See amid(st?) the winter's snow
Born for us on earth below
See the tender Lamb appears
Promised from the eternal years"

"among" wouldn't sound right though would it?

But shouldn't a carol be 'literary'?
Actually I think the whole concept is wrong- he would be sleeping 'on' the hay surely- 'among' seems to imply he is surrounded by it. Anyway it isn't a nice word is it? Strangely enough "amongst" sounds slightly better and I agree 'amidst' has definitely a better sound.
But I suppose you'd better sing the words that were written!

Among or amid – surrounded by – are both correct. Amongst and amidst are both archaic and, some would argue, literary.

I think the noun needs to be plural. Sleeps among or amongst the sheep, cows, whatever.