Audio clip change via script,changing audio clip in audio source via script

What's important is what audio you want to play. You can use a general AudioSource to start playing a sound that is mixed with other sounds. Add more audio sources for sounds that are continuous and require start/stop playback.

 public AudioClip clipFire; //set in inspector to a clip
 AudioSource generalAS;
 AudioSource engineAS;
 void Start()
 {
     //this:
     generalAS = GetComponent<AudioSource>();
     //or this for multiple audio sources:
     foreach (AudioSource aSource in GetComponents<AudioSource>())
     { //we have 2 audiosources
         if (aSource.clip!=null && aSource.clip.name.Equals("engine"))
             engineAS = aSource;
         else
             generalAS = aSource;
     }
 }
 void Update()
 {
     if(bulletFire) generalAS.PlayOneShot(clipFire);
     if(startEngine) engineAS.Play();
     if(stopEngine) engineAS.Stop();
 }