Solfeggio (1963) written by Arvo Pärt in the simple scale of C major is an a cappella SATB choir piece. Scored Largo, the voices enter in different octaves successively with notes from the tone row, forming vertical sound clusters. The lyrics are simply the syllable names of the notes: do, re, mi, fa, sol, la, si.
I had the pleasure of singing the bass part with Die Zeitlupe at a concert in St. Michaelkirche, Bremen-Grohn in April 2006.
Solfeggio can be found in 4 Voices, a choir book with some 200 songs.