Header Youtube vid not playing sound – please help!

Hi there, thanks for a great theme. However the youtube vid we have embedded as our header vid on the home page does not play sound. The website is http://www.diminished5th.nz (The vid on youtube does have sound, and we have also included the video on another page within the website, and that has the sound no problem. ) Can you help? I have looked at the raw code on the page <iframe id="wp-custom-header-video" frameborder="0" allowfullscreen="1" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" title="YouTube video player" width="1920" height="1080" src="https://www.youtube.com/embed/MRzBCqep8iE?autoplay=1&controls=0&disablekb=1&fs=0&iv_load_policy=3&loop=1&modestbranding=1&playsinline=1&rel=0&showinfo=0&enablejsapi=1&origin=http%3A%2F%2Fwww.diminished5th.nz&widgetid=1"></iframe> The link on its own plays the video with sound no problem. So is there something in your code somewhere on the page that automatically mutes embedded videos? many thanks Bill