It will be that the sound has been deactivated on the Parent Unit - in such instances the sound icon will then disappear from the display on the Parent Unit.
The sound can accidentally be activated/deactivated when you change the Movement and / or the TIC feature.
If you change one of these settings then scroll through the features again the sound icon will show in the display and you can activate/deactivate this function at this time.
For advice on changing the settings kindly refer to page 11 in the user manual. If you no longer have this see the following link to directed to the manual section of the Angelcare UK Help Centre: