Symbol ! audio plays once?Why?

var Audio : AudioClip;

private var Som = false;
function OnTriggerEnter(){	  	

 if (!Som) {
    Som = true;			

//So can you guys explain what is that symbol?

The “!” character is often called a “pling” or indeed an “exclamation point” or “exclamation mark.”

In computing languages, it applies to booleans and means "not"

So if you have a boolean called x that is true, then

  ! x

would be false.

Hopefully that makes sense. “!” gives you the opposite of a boolean. If true, then false. If false, then true.

the ontriggerEnter is called only once when you enter a collider whose istrigger property is true in inspector.
thats why it is played Once and not in a loop.