You can record the greeting for your Auto Attendant in *.WAV or *.OGG format. Here are the required specifications for custom sound files:

  • mono
  • WAV/OGG, u-Law
  • 64kbits/s
  • 8000Hz sample rate

If you would like to restore the default greetings, you can download them from here:
Default Sounds.

The easiest way to record a greeting in the desired format is to use one of the 3rd-party sound recorders like Audacity which do support the above format.