This one one I've always been trying to figure-out. Morricone's original recording was always hard to for me to pin-down, though the more accentuated percussion on Silva Screen's "Cinema Choral Classics" sounds like 3/4, or maybe 12/8. Has anyone ever seen the score to know for sure?
Just listening to the OST, and it sounds like a "normal" 4/4 to me. In the second half, there might be one or two 2/4 measures, in addition to some off-beat percussive accents. The hand drums that start at 0:47 appear to be doing their own, rhythm-independent pattern, as does the choir (which towards the end of the piece does strongly suggest a 3/4, I agree).
Someone with actual musical knowledge should easily pin this one down. 'ang on, Imma yell for David Coscina...
As a non-musician with some proficiency in the technical aspects, I have always taken this to be a polyrhythmic piece, with essentially three different voices taking place at the same time, each in its own meter. There is the overall chorale theme, with choir and harpsichord, which begins the piece and plays throughout, with drums accompanying midway at a faster rhythm but still congruent tempo. I hear this section as 4/4 at a slow tempo. Then there is the oboe "Gabriel's theme" voice, which seems to enter at its own tempo and rhythm, playing independently over the chorale. No idea the time signature of this very long-lined theme but its phrasing is accented entirely independent of the chorale theme. Lastly, you get the get woodwind theme with flute which has an entirely different rhythmic character, added about 2/3 of the way through. This theme appears to me to have an irregular meter with bars of different time signatures depending on where you are at. Overall, these three themes play simultaneously and at times incongruently, both in terms of rhythm and harmony, but the result is a kind of gloriously chaotic union of sound and purpose.
Someone with a better grasp of theory may be able to say whether the combination is truly polyrhythmic or whether the piece is basically all in one rhythm, with different tempi and accents. My ears hear the former but I don't have the score in front of me.
I think there are good reasons why this piece is so widely praised as among Morricone's best. This single cue condenses nearly every major theme of the score, not just in an old-style overture, but played all at once despite each theme having entirely its own rhythmic character. How the man conceived of all this and managed to notate it in a form that could actually be performed and understood is a source of wonder to me.
It's in 4/4 and 12/8 (with each quarter note equalling a dotted quarter note for the choir, percussion etc), or just 4/4 with 8th note triplets (further subdivided into 16th note triplets for the bongos).
I haven't studied the score, so this is just guesswork.