Tuba Mirum, from W.A. Mozart's "Requiem Mass", is used in virtually all orchestral auditions as a measure of a player's legato style and musically expressive capabilities. We are often asked why Mozart used the trombone as a solo instrument in this movement, but a quick perusal of sacred choral works of the pre-classic and baroque periods reveals plenty of precedent for this choice. The excerpt should be practiced at a variety of tempos centering around quarter note = 82 . I have performed it as fast as half note = 54, but this would certainly not be the norm.
One of the primary concerns in the opening phrase is where to breathe. Ideally, the phrase would be performed in one breath, but in most instances this seems not to be practical. In performance, the placement of the breath may very likely be determined by such factors as tempo, size of the hall, and/or the conductor's interpretation, but in an audition the decision is yours. Frequently recommended places to breathe are at the end of the first measure, in the second measure between the third and fourth beats, or at the end of the second measure before the low b flat in the third bar. Regardless of your choice, the breath should be taken in such a way as to disrupt the musical line as little as possible.
One idea for the character of the opening phrase may be derived from a story told by Carl Lenthe, an American trombonist who has played this piece many times in Europe. On one occasion, the conductor asked that the Tuba Mirum be played without pause after the particularly forceful ending of the Dies Irae. In this way the "Last Trumpet" sounds as a direct result of the "Wrath of God"! Consider the possibilities!
Most players are aware that the rest in the fourth measure is often used to determine a person's ability to maintain a steady pulse. Mentally subdividing eighth notes through the first phrase and then through the rest should assist you in coming in correctly in the next passage. For the remainder of the excerpt, it is important to remember that the trombone part is an obbligato voice to the vocal solo. It should be played as beautifully and musically expressive as possible. The expression should be the result of musical inflection and the judicious use of dynamics and should at all times serve to enhance the vocal line. In the interest of fine tone quality and intonation, the use of alternate positions should be avoided even though several will suggest themselves to you. Two exceptions to this rule are the f on beat three in measure six, which when taken in sharp fourth position will help to match the quality of the tone to the preceding g flat, and the b flat in measure thirteen on beat two which when taken in sharp fifth position will help insure against any unwanted glissando.
The Tuba Mirum is among the most beautiful excerpts for trombone in the orchestral repertoire. Listen to as many recordings of it as you can get your hands on and practice it frequently. Good luck!