The orchestra and chorus are mute. Nothing is heard but the trumpet. The trumpet sounds as a single note played softly. To suggest the approach from a vast distance Verdi makes use of four extra trumpets behind the scenes. The trumpet calls grow stronger and stronger. Verdi keeps to the dark ambience of A flat minor throughout and generates this entire passage from one cell, expanding the semiquavers into triplets where appropriate.
When the long-awaited tonic chord is reached full brass, bassoons and timpani weigh in, giving the music an almost thematic substance. The bass soloist rejoins with 'Tuba mirum spargens sonum' over full orchestral chord to which chromatically descending strings give the feeling of a universe falling apart. The Chorus echo his words while the air resounds with brass fanfares.

Tuba mirum - realaudio

The pace quickens as the two blocks of sound - brass and tutti each with chorus - alternate and intermingle, until the music stops short with a terrified shriek.

In the quietness that follows the bass contemplates as though peering into an abyss the amazement of death and nature as every creature is called to account. Strings repeat an obsessive ostinato of jagged contour, punctuated by a beat on the bass drum, the skin now slackened to suggest infinite depth. On the last two syllables of 'responsura' (Creation's terrified 'Answer' to its judge.) there is a unison blast from lower woodwind and horns, a device suggesting utter terror in the 19th century opera and in Verdi's musical vocabulary.
The canvas becomes increasingly empty; The bass unable to get beyond the word 'Mors'. Here is all the horror of the void.

Mors stupebit - notice the bass in his last utterances