I was wondering which form of communication would be best in regards to analogue or digital.
Wouldn't that depend on whether you want to communicate analog or digital data, and what you want to communicate with?
Basically, there are couple of sensors that detects (eg: motion, moisture, temperature)
For what purposes? I would expect the baby to be exposed to the same atmospheric conditions that you are. If you are cold, put a blanket on the baby. If you are hot, take it off. If you are wet, get out of the rain, and take the baby with you.
also a voice detection system which turns on the system when baby cries.
You'll hear it, don't worry. From 3 blocks away, you'll hear it.
So I want to know which method of communication would be best for this system and also can you please provide some reasons.
Can't make any recommendations, because you haven't said what is communicating with what, and what data is being exchanged.