If you've enabled the Read&Write for iOS keyboard and there is no sound when using features to read text, etc. if Silent Mode is On, that will disable speech for the Read&Write for iOS keyboard.
On Newer iOS devices
Swipe down from the top right of your screen and verify that Silent Mode is set to Off
Silent Mode Off
Silent Mode On (Sound will not work with this setting)
On Older iOS devices
Check to make sure the ringer/silent switch is enabled