User Configuration > Administrative Templates > Microsoft Word 2016
User
HKCU\software\policies\microsoft\office\16.0\word\options
This policy prevents Read Aloud from reading text when app goes in background. If you enable this policy setting, Read Aloud will be disabled and will stop reading the text when app is sent to background. If you disable or do not configure this policy setting, Read Aloud will continue reading the text when app moves to background and user will be able to control Read Aloud via notification.