Its a button, ya know...Depends, sometimes.
System
Music
Mood
There is not always a button.Its a button, ya know...
Button envy.There is not always a button.

That suggests you have a complete misunderstanding of what Fletcher-Munson curves are all about, and it's a common misconception.If it has no loudness at all, then I'll add some using a mastering processor to get closer to a Fletcher-Munson curve.