How do I prevent audio from restarting if it's being asked to play again?

Basically I'm trying to add sound to my fully automatic M4. I have a sound file of a single shot from an M4 which I'm using. I made a script that tells the sound to play every few seconds as long as mouse button 1 is down. The problem is that instead of the sound playing out fully with the new one playing on top of it a few seconds later, the sound just restarts constantly without playing out fully. It works fine only if my weapon is semi auto with a RoF longer than the sound file, but my weapon has a high RoF. Here is the script:

 #pragma strict
 var gunSmoke : ParticleSystem;
 var isFiring = false;
 
 function Update () 
 {
     if (Input.GetMouseButtonDown(0))
     {
         isFiring = true;    
         InvokeRepeating("ShotSound",0,0.3); //This also acts as RoF
         gunSmoke.Play();
     }
     if (Input.GetMouseButtonUp(0))
     {
         isFiring = false;
         CancelInvoke();
         gunSmoke.Stop();
     }
 }
 
 function ShotSound ()
 {
     if (isFiring == true)
     {
         audio.Play();
     }
 }