Audition song, hmmm?
Well, I personally would say first the thing I tell everybody - go with something YOU like. If you don't like the song it's really obvious to anybody listening; it just doesn't sound the same, even if you sing it through perfectly.
Now that I've said that though...I use this site tons and tons, they're really useful for basic tips on what TO choose if nothing anybody recommends strikes your fancy...
http://www.musicaltheatreaudition.com/joomla/Also, check out musicnotes.com after you've found a song - it's about 5.00 and you get an instant download of the sheet music, plus it can be transposed and played with on the site. Pretty useful!
Um, actual song-wise? I'd say stay away from really overly popular songs. Stuff from Wicked or Phantom or Cats are nice, but you run the risk of not really sticking out.
Right now, I can think of a couple off the top of my head:
"Somewhere That's Green" from Little Shop of Horrors (for some reason my song book has this song written REALLY high - there's a C two octaves above middle C; watch out for that)
"Girl I Mean To Be" from Secret Garden (this is a nice mid-range song, and I think you could actually fit most of it in the 16 bars)
"No One Knows Who I Am" from Jekyll and Hyde (another fairly short one)
Try checking out sheetmusicplus.com for "soprano" and you'll get some Audition books - they list the songs in the book, so reading that might help.
I hope I've helped you somewhat. Good luck!