You can record the greeting for your Auto Attendant in *.WAV or *.OGG format. Here are the required specifications for custom sound files:
- WAV/OGG, u-Law
- 8000Hz sample rate
If you would like to restore the default greetings, you can download them from here:
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.