I agree that a half hour is a really long time for a private audition. I do work at theater that has always done private auditions. For that theater it is expected that there will be prepared music and memorized monologues. I do ten minutes which seems to be fine.  Getting people to audition is not a problem. For call backs I do have everyone come at the same time, and I let them know how long I think they are going to be there.

We have group auditions. As was said before, I also want to see how people interact with each other.  Another thing to think about in Community Theater is that you do get a lot of first timers, and if they cannot get up on stage with people they might be acting with for auditions, how are they going to get up on stage in front of an audience?  We don't ask for resumes or head shots, we have a basic audition form to fill out and then take a picture of them at the audition.  We don't do musicals very often, so it's mostly cold reading for most of our plays.  However if someone does bring a prepared monologue, we let them do it.

When we auditioned "Christmas Carol", we had board members act as "readers", and read the other role(s) in the scene one actor was auditioning for.  If we had enough actors (and we never do), we might do a callback, and see how promising actors work together.  For a musical, the actor is expected to bring a song and sing a capella (usually they don't sing as well with piano, although we had an accompanist).  I think having someone to play off (reader) helped people audition, and the director did not have to focus on more than one person at a time (our directors are not very experienced).  Our only dud casting choice was an actress who could not project (we auditioned in a rehearsal hall).

Sides are cuttings from the script. They should show the character's range. I usually have  two scenes for each character.  One where they control the scene and one where they support the scene. I have the sides available for actors to look at before auditons start.

In my opinion, Callbacks are indispensable.

At first read, actors can be nervous, distracted, etc. Maybe they had trouble finding the theater or parking and didn't get to spend as much time reading the audition piece. Maybe they were frazzled from a long day at work, or just had a fight with their spouse -- maybe they forgot their reading glasses or their medication.

There are a million reasons why a single audition isn't a satisfactory example of an actor's abilities.   A callback gives those concerned a second chance. Plus, the idea is not to call back EVERYBODY (that would be a waste of time), but only the strongest contenders for the meatiest roles.

In my city, we are never short for actors so the first audition is merely to winnow out those that just aren't right for the script -- too old, too young, too tall, too short -- whatever. It also tends to weed out the wannabe's and loose screws who are enamored with the "glamour" of acting, but don't realize it takes LOTS of rehearsal and hard work to get there. ("You mean I have to come to EVERY rehearsal?!" and "I don't want a costume that will make me look ugly!" are two favorite quotes overheard at the AUDITION!)

Callbacks also give the director a chance to mix and match actors -- to see who has "chemistry" with each other; to see who can work together without overpowering each other; and more simply, who just "looks right" together. It also gives the director a chance to see which actors can take direction -- a skill that is immensely desired.

Musical auditions are to separate the "Actors who sing" from the "Singers who act" and, frankly, to learn who does neither.   In many of the theaters in my town (both community and professional) it is not uncommon to hold the singing auditions FIRST. The actors who CAN'T sing aren't even called back to read for parts. This is to insure the entire cast is populated with strong voices. Many actors (myself included) don't even bother auditioning for musicals because they know they'll never it make it past the first cut.

I agree with Linda S. Casting is the hardest part of a show. It's like buying seeds through the mail. The package looks right and they promise to deliver on time, but you never know what's gonna sprout until the harvest.