Bert Coules wrote:

I thought that digital files (of whatever format) reproduced exactly
from generation to generation with no deterioration at all.

If you copy the file over and over, it'll retain the same quality, but
if you keep-re-encoding it, e.g. to mix in other channels each time,
you'll get gradually worse, if you can arrange it so you mix in all the
extra channels just once, so there's effectively only one extra
generation you'll get away without problems.