Hi William,
You would just change this line:
GetComponent<AudioSource>().PlayOneShot(thunderSounds[Random.Range(0, thunderSounds.Length)], Global.GameSound);
To use the FMOD Unity integration:
FMODUnity.RuntimeManager.PlayOneShot("event:/path/to/event");
You can find more information on using FMOD with Unity in our documentation, and we have a few YouTube series on this as well:
Thanks,
Richard