It is perhaps surprising that it's taken this long for someone to think of tackling In C vocally. There are some challenges. Other versions I have heard use a wide range of timbres and so it is quite easy to hear the distinct sections (snippets, whatever) interacting with each other. By only using marimba and voice, the variations are less obvious. Paul Hillier has used this to his advantage, by using quite a bit of unison singing, making it sound in some ways like plainchant. And the use of syllables rather than purely sang notes adds some variety. At the same time, according to the sleeve notes, the singers have employ a number of variations on the original motifs. Both the unison singing and the variations require more arrangement than might be expected of a piece that makes a virtue of individuality. The overall effect is somewhat 'churchy' and dramatic (especially the surging voices at the start). So, while this is definitely and recognisably 'In C', it is also something new and fascinating.