Thanks folks for the info and the links. Already found some stuff here I didn't even know existed!
I'd like to be a little more clear though than I was in my original post which was a bit muddy.
In the theme from GBU, it sounds to me like the male vocals are saying "You-are-too-grateful". The Spaghetti Western Orchestra uses the words "Go-to-echo", "Go-to-rainbow" or something similar sounding in their GBU theme. Maybe others hear something else.
I was wondering if anyone knows if the actual vocals, as heard in the film itself, are actual words or just gutteral sounds that aren't actually words at all. Much like in Ecstacy of Gold where the female opera singer is just doing a series of ooohs, aaahs and other sounds.
As before, this same question also applies to the theme from For a Few Dollars More.
Again, thanks for all.