Whatever you do, have a process and stick with it.

Even if there is only one auditionee, that student completing the process sends a message that their position is earned by meeting your standard for leadership. I would communicate they must complete the process to a satisfactory degree to EARN the privilege of serving as Drum Major.

Having a drum major who feels entitled to the position can cause a lot of damage to a marching season and the morale of the program. Submitting to your process also communicates to the other students that you support the DM and their "authority" comes from you, not just being the only candidate.