Tira here, from Shopify Support. I'm happy to shed some light on this for you.
Thank you for providing your theme name. I understand you want to add a video to your slideshow, have your videos automatically play on your mobile device, and make sure the audio plays on the browser. Is that right?
By default, the slideshow will not play on mobile devices depending on the browser being used to access your store. You can add an image as a placeholder for when this happens. Here's a screenshot from the theme editor on my test store:
To access this, you can login to your Shopify admin, click on Online Store > Themes, then click on the Customize button next to the theme you wish to edit. Under the "Sections" area, click on "Slideshow" and you will find the options to add a video or image to your slideshow.
Sometimes popular browsers can automatically turn off videos. Here's an article I found online that may help: Google Chrome will start blocking noisy autoplay videos in January. Customers will need to press the "play" button to play these videos when this happens.
If sound is important to your video, I recommend using the "Video" section of the Narrative theme. This will make sure your videos play when someone lands on your store. Check out the steps to add a video to your theme, here: Video - Narrative.
Let me know if this helps you accomplish this!