They state that these are cricket chirps only and there are no musical interludes or voiceovers and that these are just recordings of crickets chirping at different sound speeds. It doesn't really explain the process of recording in much detail. It does sound like an angelic choir. That is, as far as we can imagine... Listen Here
"Cricket" from the medieval bestiary:
other names: Crisnon, Gresillon, Grillon, Krillum
A creature that likes to sing, forgetting all else.
As described by Pierre de Beauvais in his Bestiaire, the cricket likes to sing so much that it forgets everything, loses its appetite, lets itself be hunted, and dies singing.
Here is St Augustine’s quotation on music:
‘Will you ever, do you think, that you need know no fear of jarring on the perfect listener’s ear? This is the way of singing God gives you; do not search for words. You cannot express in words the sentiments which please God: so praise Him with your jubilant singing. This is fine praise of God, when you sing with jubilation. You ask, ‘What is singing with jubilation?’ It means to realize that words are not enough to espress what we are singing in our hearts. At the harvest, in the vineyard, whenever men must labour hard, they begin with songs whose words express their joy. But when their joy brims over and words are not enough, they abandon even this coherence and give themselves up to the sheer sound of singing. What is this jubilation, this exultant song? It is the melody that means that our hearts are bursting with feelings that words cannot express. And to whom does this jubilation most belong? Surely to God who is unutterable. And does not unutterable mean what cannot be uttered? If words will not come and you may not remain silent, what else can you do but let the melody soar? What else when the rejoicing heart has now words and the immensity of your joys will not be imprisoned in speech? What else but “sing out with jubilation”?’
St Augustine, On Psalm 32, Sermon 1, 7-8; quoted in the Office of Readings for the Feast of St Cecilia, November 22nd