Please keep in mind that only office administrators are able to make changes to auto attendants.

-Text to speech: It will let you type a message and choose a voice to read the message you've set as a greeting.
-Upload: Lets you browse the local computer to upload a recorded file in .wav/.mp3 format.
-Record: Once you type in an extension or even an external number, a call will be given out with the recording prompt. Once finished, it will play the recorded message on the auto attendant greeting.
Note: Intro greeting for auto attendant can be set up the same way. It will be the message played before the menu prompt message.
Generally, people would like to keep one message played throughout the auto attendant. If you think you will frequently change the menu options, setting up an intro greeting might be helpful. This way, one message will always be the same such as "Thank you for calling BlueCloud." and then the message afterward can be changed depending on your menu options.