If it's a professional audition, are you sure you'll be allowed to sing an entire song? Or will they just want your best 8 to 16 bars? If the latter, the song almost doesn't matter except it should be something that demonstrates your range is right for the role.
If you get to sing a full song and want something nobody else will be doing, look at "Letters" from BAKER STREET. Not a great score otherwise, but "Letters" is a lot of fun: a late-19th century actress describes the letters she gets from gentlemen fans, imitating each letter writer as she quotes from his letter. The listener doesn't have to know the show to "get" the song. If you have Glinda's range, this might make a good audition song to learn and have handy for call backs. You can download the song by looking up the OBCR on iTunes. (The full song clocks in at 5 minutes, but an arranger could probably shorten it for you by omitting one of the letters.)
Speaking of Inga Swenson, something from 110 IN THE SHADE might also work. "Raunchy" seems the logical choice since it is relatively short and, actually, not really very raunchy. The joke is that the character's idea of raunchy behavior is actually pretty innocent.
Whatever you do, DO NOT sing "Glitter and Be Gay" unless you have really worked on the song and it sits perfectly on your voice. That song has been sung too well by Barbara Cook, Kristin Chenowith and several opera singers; a less than perfect version is painful to the ear. (And is the reason I don't own a recording of the Hal Prince revival from the 1970s, even though I own five other versions.)
Whatever you decide, break a leg!
