According to this post Best Baby Monitor With Long Range Audio and 2 Cameras reviewed:
Parents always feel the need to be near enough to their sleeping babies to hear them cry. But sometimes, work needs to be done outside, your other kids want to go out and play, or you’d like to catch up with the neighbors. Maybe you just need to know how to find the best baby monitor for a big house, which will have a strong and clear signal.
Before you begin your search, you need to know if you prefer to have an audio or video monitor. The models available as audio only do out-perform the video baby monitors in range. For some families, it may be a good investment to purchase an inexpensive audio monitor and a separate video monitor, which you will use solely indoors. This really depends on the amount of distance you will be traveling away from the house. If you have a large yard or want to go next door, you may have to do this.
Once you have an idea of what you are looking for, you should be able to find a monitor that will suit your needs. Be sure to know the distance in feet you will be going away from your child’s bedroom, and account for the fact that the distance lessens somewhat with interior walls breaking up the connection. Be prepared to extend the baby monitor distance you estimate you will need to account for this.
Long range baby monitors for a large home will range in price from $40 up to $200. This would include one receiver, which is ideal for one child. Often extra receivers can be purchased when needed.
Of course, adding video with night vision, LCD screens and wireless technology makes the price higher. Having audio alone, as all models will provide you, does ensure the safety of your baby if you are close by and can reach them within minutes when they begin to stir.
Keeping an eye on your baby when you can’t be in the room can be great. Some models also offer two-way communication. Although this does not seem important for parents of an infant, it actually can be. It allows you to sing a sweet lullaby to your baby from another room. It may calm them down while you finish dinner preparation or fold the laundry. And as parent, we know that every second counts. So, it may just be one of those extras you can really appreciate once you know how you can use it.