I'd check with the company you're paying the royalties to.  Samuel French, Dramatists, etc.

I'm not saying that I know the specifics of the law but I have had call-back to a few smaller professional theaters where I was supplied some "sides" of music and readings over the internet in e-mail from the show. My feeling is that it seems unlikely that these theaters would risk putting themselves at odds with licensing companies unless they could legally do it. And these theaters undeniably had already obtained production rights for the show . But again, I don't know the specific details of copyright laws. But only that I know some theaters that do it.   

You don't list where you are, and this kind of issue varies from country to country.

BUT, if you're in the U.S., you should be clear on this under the terms of Fair Use, as listed above, as long as the distribution is limited (mail your auditioners a URL that's not linked to publicly), confined to a reasonably small amount of the script, and you've secured performance rights and own a copy legally.

IANAL, but even copyright lawyers will tell you that Fair Use is a very grey field.  Fortunately, intent is usually a large part of any decision, as is profit.  I'd say if it came to a head, odds would be strongly on your side, but chances are that it'd never get that fair -- because:

1. The publisher would probably never know;
2. If they did, they probably wouldn't care;
3. If they did, they'd have to win; and
4. If they did, the damages would probably be so small as to make their action not worthwhile.

It occurred to me that in my last post I failed to provide any explanation of the fair use doctrine in copyright law. (I realize many may know this, but many also don't, so if you do I apologize.) Basically, fair use allows people to use copyright protected material without needing to obtain permission from the copyright holder. Whether a use constitutes fair use is evaluated on a case by case basis based upon several factors. http://www.copyright.gov/fls/fl102.html

The fact that other theaters do this certainly works in the favor of fair use, however, it comes down to a totality of the circumstances. Including those factors I mentioned above. It's also worth mentioning that in the case of the theater company I am directing for, use of script portions for auditions is a standard part of their contract with the playwright. (They do new works so they have the luxury of using their own contracts.)

David raises an important point. It is certainly an evaluation for your theater company to make as to whether they risk being sued.

David, I do have to step in regarding your second point though. This is not to start a fight , but because it might also help shed some light on what kind of lines exist in this area.

As far as copyright law is concerned there is a HUGE difference in copyright law between making an archival copy and selling copies in the lobby. If the rights do not include permission to make an archival copy, a case can generally be made that an archival copy falls under fair use. (As always the specific circumstances may effect this) The difference here is that with the copies in the lobby:
A. You are making several copies and distributing them to the public which is a right reserved to the copyright holder. With an archive copy you are making one one copy, and your not going to be broadcasting it to many audiences. (Come to think of it, in all the years I've done theater I don't think I have actually ever seen any of the archived copies of shows I've been in. Or the pictures from photocall come to think of it)
B. You are selling these copies (presumably for a profit) also a right reserved to the copyright holder. Obviously with the archival copy you are not selling the work

It is extremely unlikely that these two differences will still protect you under fair use.

In some ways you are correct. It's very possible that even giving copies away in the lobby for free will be enough to not meet the fair use requirements.

I mentioned the profit aspect because the first prong of the fair use analysis specifically looks at whether the use is commercial in nature. Courts generally view this as whether a profit is being made. Although there are some exceptions, in general if a profit is being made, that is a nail in the coffin for any fair use claim.