CMX Stingray 888 FM 4GB -> volume automatically lowers after some time

As I wrote in one of my last entries, I own a CMX Stingray 888 FM MP3-Player with 4 GB

The last day's I experienced the situation, that I adjusted the volume during playing a podcast.
During the following time period the volume automatically decreased and kept steady at 22.

This is a problem, when listening when strolling beside a road where buses and trucks drive
by and you nearly hear nothing at this volume level.

Today I found, what I have changed to cause the behavior.

I have activated in System / ear protection, the protection level. With this active, the
volume is automatically adjusted.

After deslecting the featuer, the volume keeps constant.