As Tanshaydar said, there are multiple ways to call functions.  Thepaleking's example is how you would go about making it so a sound plays when a character enters an area.  
Check the script functions page for more information, but here's the general format for what you are trying to do:
/*The 1 signifies that the function will be called upon entering the area.  -1 means it will be called upon leaving the area.  0 means both.
*/
void OnStart()
{
   AddEntityCollideCallback("Player", "NameOfArea", "NameOfFunction", "DestroyCallbackOnUse(true or false)", 1);  
}
void NameOfFunction(string &in asParent, string &in asChild, int alState)
{
    PlaySoundAtEntity("soundname", "soundfile", "entity", "fadetime", "savesound")
}
Note that you should probably keep "savesound" as false, because it will loop otherwise.  Soundname isn't important, just pick something that no other sound uses.