Click on the gear-icon -> Personal settings and Telephony
To record a Voicemail Sound file:
Choose Sound file under Message and then New Sound file
You can upload an existing sound file or record a new voice message. In this case, we want to record a new message.
Name your sound file, click on Record a new message, and when you have recorded your message, you just click on Save, and it's active.
You can also use Text to Speech.
Instead of recording a new message, click on Text-to-Speec (TTS)
Choose the language and voice you want to use.
Enter your message and press the play button to listen.