Diagnosis
The "Out of Range" alert is a safety feature, but its default volume is tied to your main volume setting, which is probably too high.
How to Fix It
You can't turn the alarm off completely, but you can lower it drastically. 1. When you're in range, turn your monitor volume down to a level where you can just barely hear normal room noise. 2. The out-of-range alarm will now be at that same, much lower volume. 3. If you need louder normal volume, you'll have to remember to turn it down before moving out of range.
If That Didn't Work
Try to stay within the monitor's operational range (about 700 ft open air, less through walls). The alarm triggers when the signal is lost for more than 25 seconds, so moving quickly between floors can sometimes avoid it.
Heads Up
If you're getting the alarm while still in the same house, it's likely interference. Manually change the channel on both units (Menu > Channel Select) to a clearer one, which will strengthen the effective range.