For those looking for a way to get a sexy, cool, neat, or any other variation of a voice for your GDM login screen, there are ways to do so.
When you go to Google.com, do a search for text to speech. You will find many websites that allow you to hear a sample of the voice. Some will also allow you to type in what you want the voice to say. For example, you can type, Welcome. Please login. and you can hear the voice say that on your speakers.
Unfortunately, not all allow you to download the sound to your computer. Therefore, you will have to find an alternative. I recommend using audacity and record the sound as it is being played on your speakers. After this is done, save the file as login.wav.
From there, you can use it as the login voice for your Linux system with GDM.
The article that will show you where the login sound can be changed can be found at http://aprivatebeach.com/blog/2008/10/ubuntu-turn-offon-the-sound/