Reported by Nikolaus Schaller, Jul 22, 2019

the twl6040 can tell the audio subsystem that there is some DC 
offset in the class D amplifiers. But this is not taken care of by 
the McASP/BSP drivers.

The result is that the speakers are driven with DC offset which has 
negative effects:
* higher energy consumption than needed
* membrane is already deflected in 0 position
* loud sound may make the membrane run to the mechanical end 
positions which gives a very scratchy noise

Solution: read out these registers and make the OMAP subtract the 
offset from the data before it is sent through the serial interface.

It may be that the AESS could handle that better, but we do not have 
AESS up and running.