I can't explain why it disappeared if it was there, but you can add the function to the Quick Access Toolbar (QAT) for convenience.
See this page:

Use the Speak text-to-speech feature to read text aloud

https://support.office.com/en-us/art...8-189d6b83333c

Click the image below to see a short video showing the setup of the Speech option (QAT) in Outlook 2016.