Hi, I'm Enzo, from Italy, i'm just bought an FA-08, great Keyboard!
The problem you're facing with regard to the headphones , it is probably caused by the fact that the connector for the headphones is just around the beam controller
Probably had the beam controller set to "expression" .
When you entered the headphone connector , your hand has inadvertently changed the volume through the beam controller .
I apologize my english.
Bye, Enzo