How can I start at a random point in an audio clip?

Rather than picking a random point in the clip you can delay the start of the clip. The easiest way to do this is to make sure the each objects AudioSource does not have "Play On Awake" enabled and delay the start of the clip through your script. Below is an example of how to assign a random delay to each creature starting to play the clip:

JavaScript Example:

 #pragma strict
 
 // Make sure there is an AudioSource
 // attached to this object
 @script RequireComponent(AudioSource)
 
 // The minimum and maximum delay
 // value in seconds
 var minDelay : float = 0.0;
 var maxDelay : float = 5.0;
 
 function Start () 
 {
     // Just to make sure we are
     // going to disable the play
     // on awake for the AudioSource
     audio.playOnAwake = false;
     
     // Make sure there is an
     // AudioClip to play before
     // continuing 
     if(audio.clip)
     {
         var delayTime : float = Random.Range(minDelay, maxDelay);
         audio.PlayDelayed(delayTime);
     }
 }

C# Example:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(AudioSource))]
 public class DelayAudio : MonoBehaviour 
 {
     
     public float minDelay = 0.0f;
     public float maxDelay = 5.0f;
     
     void Start () 
     {
         
         // Just to make sure we are
         // going to disable the play
         // on awake for the AudioSource
         audio.playOnAwake = false;
     
         // Make sure there is an
         // AudioClip to play before
         // continuing 
         if(audio.clip != null)
         {
             // Get a random number between
             // our min and max delays
             float delayTime = Random.Range(minDelay, maxDelay);
             // Play the audio with a delay
             audio.PlayDelayed(delayTime);
         }
     }
     
 }

The scripts above have a maximum delay of 5 seconds by default, but you can easily change that to suit your needs.

EDIT: Corrected conditional check for audio clip in C# code to make it check for null reference.