The voices were not filtered out. They were recorded live with the orchestra and what you are hearing is a pick up from the orchestra microphone. The vocals were on a separate track which was left out of the isolated track.
Neil
It's from 1/4" transfers of the original recordings. I haven't heard the final track yet on the disc, but I know it was cut to the picture, and therefore presents shorter versions of some of the songs. The newly issued Kritzerland CD presents the songs complete, as recorded.
Neil