Go to the number settings (1. step) in https://manage.yodel.io/ and then click on one of your phone numbers (2. step).
Scroll down to the option "Welcome Message":
Select "READ MESSAGE" if you want to automatically read a message to your callers. You can set the language of the voice at the top of the phone number page ("Language for announcements"). Click into the text field to change the message accordingly.
Select "PLAY AUDIO FILE" if you want to play a MP3 file to your callers. You can upload the file directly in the web browser and also preview it with the play button.