When you add more than one Nursery/Camera Unit, it's not possible to hear sound from both rooms at the same time through the Parent Unit.
Also, the monitor will not automatically switch to the Nursery/Camera Unit where the sound is coming from.
You can manually change which Nursery Unit you wish to hear sound from on the Parent Unit when you are in split screen mode:
Press the Nursery /Camera Unit Sound icon on the bottom left button on the Parent Unit:
The unit that is MUTED will show: next to the Nursery/Camera Unit name (CAM1 or CAM2)