You can use Text to speech or upload mp3 recording as IVR welcome greeting
In the left panel of App, you will see menu "upload greeting" click on this after that click on plus + sign, here you can select mp3 file. once upload complete you will see your uploaded greeting.
Please make sure that mp3 file name has no space between name for example.
my recording.mp3 is not a good name, use myrecording.mp3 instead.
Text to Speech
In left panel, You can see Text to Speech menu, Click on that and you can write your IVR greeting here. When callers will call they will hear greeting.