If you have any metal in your mouth over time enzymes in your saliva can crystallize on the metal and the vibrations from AM radio frequencies can be "heard" in your head.

When I was a teenager this happened about the 3rd year in with my braces ... I kept hearing spanish voices and tejano music if it was REAL quiet.

Found out the cause totally by accident because I felt like I was going insane and while getting my braces tightened at the orthodontist one visit it was REAL quiet and I just suddenly exclaimed, "Why in the hell do I keep hearing mexican music?!?!" because no one was actually in the room with me.

My dental assistant heard me because I exclaimed it louder than I meant to and they changed all the hardware out. Made it stop.

So, if you have any metal fillings or braces, that might well be the cause.