You can access the Audio Manager at Edit-->Project Settings-->Audio; check to see that the Doppler Scale is 1. Not sure if that could be it, but if the doppler effect was exaggerated, I wonder if it might cause something like this.

I am not sure, but I think it plays multiple times the same audio file. If you have the audio activated by a trigger, don't activate it onTriggerStay but onTriggerEnter ... or avoid to play it every frame from function Update().

var voiceOver : AudioClip;

function OnTriggerEnter (hit:Collider)
{
     if(hit.gameObject.tag == "speak" )
      {

    audio.PlayOneShot(voiceOver);  

       }

}

drag this .js script in your first person controller and assign a sound in the script from the inspector, make sure that you have an audio source in your controller and add a trigger tagged speak. And it should work.
you might want to add a timer so the sound won't play again if you enter the trigger multiple times. You can do things like that by pressing buttons on your keyboard or activate them when you press a GUI button .