I agree with Kibitzer.  When I was a student at Colorado State University, they frequently required monologs at auditions.  Consequently, only people already involved in theatre/journalism/performing arts typically got cast.  I do not know how to select and prepare a monolog properly, not having the background.  I guess it might depend on your directing style, since I can imagine someone with a monolog they had prepared with a lot of coaching auditioning well, but being afraid to improvise at all.  Our last audition, we did cold readings, with members of the board acting as "readers", so that only one actor at a time was being evaluated.  Having more than one actor read at a time seemed to be confusing.  We usually have trouble getting enough actors; of course one would run auditions differently if one had too many actors.

  I think if you consider this article about auditions.
This was produced by AACT ;-
http://www.aact.org/documents/AUD1.pdf

  Which is line & almost the same as how I hold auditions.
Which I believe is what we should be about!
This may help.