There was something inside the earliest humanity that could only have been expressed through music and other arts, and humanity was obsessed with expressing whatever that was. For so many reasons, especially humanity's early obsession with the sky and stars, I belive that humanity felt connected to the univese. And music was a call to all existence that we too belonged. And that to me is spirituality.
I understand you. I consider many didgeridoo Australian music spiritual.... Pygmies chorus too ...
By spirituality i had not suggested only Christianity, but any inside deeply moving relation with God or Nature as sacred...
Your observation about Mozart going from genius to deeply spiritual is my observation too ..
But the relation with Nature which is sacred music is different from Hildegard Of Bingen or Tallis moving spirituality toward God. But the other day i was listening Russian female chorus performing pilgrim songs ( song suggesting walk and prayers) it was very near the pygmies songs ... The relation between men of the same tribe and walking and praying or singing in Nature is sacred music... It is why i loved it...
i cannot listen only geniuses composers or musicians... I need also spiritually, sacred Christian, Buddhist or African or Indian devotion music...
Music without spirituality cannot fulfill all my musical needs so genius it is...
Poetry is also bordeline to the sacred or the spirit or the religious...
Think about Leonard Cohen singing about Joan of Arc...
Or even Dylan singing "murder no foul" is a sacred musical event in the US history...
Music without spirituality or poetry is not very interesting for me...
The 8th book of madrigals of Monteverdi is pure poetry...
Poetry gives us a minimum of spirituality...