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.