Below are a few basic things worth considering when starting your work on Tuba Mirum for an audition.
1. Acceptable tempo usually falls between 1/4=72-80. I think playing at 1/4=76 puts you in a truly comfortable spot that will be acceptable to audition committees.
2. Starting with the first three bars, aim for a sound that is comfortably full. Perhaps a mf or pocoF is good. Go for very clear, consistent articulations that give you the biggest sound at the front of the note without a hint of swelling. I like to think of all of these notes being both tenuto and marcato. Shape the phrase so the musical peak is the D at the beginning of bar two with the energy relaxing subtly thereafter. If you can play the appropriate dynamic and do it all in one breath, GREAT! If not, take a breath at the end of the first bar and shape the end of the F in a way that suspends the energy of the phrase over the breath. To practice this shape, play the first three notes on one breath giving the slightest crescendo into the D. Then try to recreate this general shape while breathing at the bar line.
3. Make sure you count the rest absolutely accurately and metronomically. In the next couple of bars, give a clear articulation on the first Bb and the slur everything else. If a natural slur is available, take it. I would recommend using all natural positions. Playing the last D of this phrase in 4 position is the one exception where I think it can be done well.
4. Don't let the fermata last too long... just long enough to let the sound from the previous phrase clear the room and then continue on to the next one. Both the F and the Eb two bars later should have a crystal clear articulation followed by the smoothest legato you can make without a smear. Put a little tenuto on the repeated Bb in the second bar of this phrase to emphasize the repeated note and provide better shape to the phrase. Breathe before the Eb and play through the end of the first arpeggio on this breath.
5. For each arpeggio, give a very subtle hairpin dynamic change. Each time you will lead to the 4th note of the arpeggiated passage, and each time should get progressively, albeit subtly, more pressing/intense.
6. Count the rest accurately again. People are real sticklers for this in auditions. Play the last phrase with a smooth legato again, only re-articulating clearly the notes that follow the high Ab's. Don't slow down or a do fabricated rubato at the end. Also, I add a slight tenuto the high Ab's because I it highlights the harmony of the phrase.
7. Play the excerpt straight through a few times with the metronome clicking eighth note subdivisions. Then move the metronome to quarter notes, then halves. Get accustomed to playing metronomically. After you feel confident that you are playing with very consistent time, I would recommend feeling this excerpt in 2. When you think in 4, it starts to sound more stagnant and boring. Two has more flow. Just don't let thinking in two suddenly speed up your tempo.
8. After the fundamentals of pitch, rhythm, clear articulation and good tone are on auto-pilot, try to focus your mental energy on the general character that you are hoping to inflect. Have faith that the careful and repetitive work you have done with the tuner, metronome and recording device will not leave you. I like to think of the first three bars having a bold, stately character and the remainder of the excerpt being as cantabile and simply beautiful as possible.
9. I would play the excerpt a few times a day and just record it. Each time check for something different. Be tempo/rhythm police one time, pitch police the next, then character police, etc. Create a checklist of all the things you would like to have present in the excerpt and don't be satisfied until everything can be found in a single take that you can reproduce consistently.
10. Good luck!