bplogic:

The reason I ask is because, to my ears, sometimes on the recordings it sounds
like faders are used to fade in and out instruments or sections to create the
effect of tonal color shifting. I think the musicians COULD do this
themselves, but it seems more likely to me that straight volumes were recorded,
then faded in and out during mixing.
BRBR

While there was probably tweaking of levels in mixdown, that is what it is
about after all, I am quite sure the basic dynamics were part of the original
recording. As I have said I was not involved with these recordings, but I have
been for several years involved with the Steve Reich Ensemble. Steve is very
precise and demanding about what he wants out of them, and they are very
capable of delivering it. Because of the very tight integration of the
percussion instruments for instance they would most likely have been recorded
all together. It is possible that the voice parts were recorded later, and
possibly the winds.

Mac