How are auditions? Who judges the auditions? Are the directors super picky about who the put in the band? How long does it take to find out if you made it or not? If you do not make it after your first audition can you audition again for the same season?
Obviously this will vary by college. Which one are you asking about?
Usually drum majors and/or the grad students will be the judges. They’ll judge posture, over exaggeration of marching technique like glide step, quickness of about face turns, how you hold your instrument, and they’ll judge you on your playing ability which doesn’t matter as much as the actual marching, but almost as much.
They’ll probably have you play some excerpts of the upcoming music for the season, and you’ll need to be able to learn and play that on your own since it’s unlikely you’ll get help from the music department there until you become a student.
Usually you just have to be absolutely inept at marching to not make it. They like having people, but you can’t be one of those prisses that doesn’t bring a working reed outside or complains that marching in a straight line is too hard.
^And even that is dependent upon the university. A university with a middling marching band follows these steps. A university with a really good marching band might have a more rigorous audition (perhaps including some sight reading). And a university where the marching band is sort of less important perhaps won’t have a formal audition at all.
Can also depend on instrument - There is often a great deal of competition for battery positions. That said…every school is a bit different on this one.