I wrote to a friend who is the owner of a wonderful planning site for Salzburg visitors: http://www.visit-salzburg.net/ Benedikt was kind enough to provide this info for you:

"For Tom: There are always programs in the daily news (e.g., 'Salzburger Nachrichten', province supplement) about ALL theatre performances/concerts on that particular day; if Tom is under 26, he can get very cheap tickets 30 mins before the beginning of performances in the Landestheater. Beyond that, I don't know of any last minute offers - standing spots are generally affordable, though. Hope this helps! Best wishes, Benedikt"

A helpful site for Vienna is http://www.wien.info/article.asp?IDArticle=9010

We have written about both Salzburg and Vienna in our Austria travelogue, which has other links as well.

Have a great trip! Linda Toronto http://www.thetravelzine.com