The LM4857 is an audio power amplifier and part of the Neo1973 audio subsystem. It's connected to the VB power rail (battery terminal voltage) for power and thus needs to be powered off separately from the main system.
The audio power output may vary as the battery discharges:
The stereo (LIN and RIN) input is connected to WM8753. The mono (MIN) input is not used.
Mixer settings starting "Amp" control it via I2C, including: