Someone may have heard it but they are too afraid to speak out, or they may have thought it was firecrackers. BTW guns are freely available on Koh Tao and there are plenty of pictures on social media of the local would-be gangsters posing with them. One theory is that the gun may have gone off by accident.
However, you would have thought that the British autopsy would have detected gunshot wounds but so far the British authorities have remained silent and the Coroner's inquest has been postponed until after the trial concludes.
Perhaps this is the important evidence. Remember the report of a female ferang running screaming?
Edit: Don't they have that covered already? IIRC they found a gun in the workers quarters. Not sure if my memory is serving me well here.