Auditioning for the Boston Symphony can be the most grueling five minutes of a musician’s career, not to mention the most expensive (orchestras don’t pay for your plane ticket or hotel room.) One slip-up, one mental lapse, and all your preparation can be for nought as you’re dismissed from behind a screen with a cursory “Thank you.” Not only that, the BSO’s music director, who ought to be the final authority on all hiring decisions, has taken to not showing up for auditions, which has many of the orchestra’s musicians upset, and candidates wondering whom they’re supposed to be trying to impress.