Depends on the your school's music program. My school tried to prevent non-music majors from finding out about auditions. My freshman year I could not figure out how to audition for the select chorale. This cost me a free trip to England. When I finally pinned the director down for an audition he said "We could have used you in England!" and I said "I would have gone if I anyone would have told me how to audition!" He said "Yeah, we try to keep audition information strictly inside the music building." "Well you shot yourself in the foot on that one."
But once you figured out HOW to get involved, they worked on merit alone for both choral and instrumental programs.
Bottom line - it can't hurt to audition. If you suck, the worst that happens is you don't get in...and you're not in now, so you've lost nothing but a bit of pride.