If you've enabled the Read&Write for iPad 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 iPad keyboard.
On Newer iPads
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 iPads
Check to make sure the ringer/silent switch is enabled