ANS system
The B777 uses ambient noise sensors in the cabin. They monitor the local sound level and filter the noise and signal to compute the signal to noise ratio. The result is used by the system to control the PA level in that zone. If memory serves there are 5 PA zones. This is in addition to the normal discrete inputs mentioned above which increase PA level for engines on, A/C speed, decompression etc. The cabin crew can turn the ANS system off if it becomes troublesome, (and it does).