I have had a search around the forums but could not find a solution to this.
Wix support info suggests that videos will not auto play on mobile (or tablet) because of data usage reasons however then suggest looking at Apple and Google developer tools.
Apple does include some reference to cading changes that could perhaps enable the videos in the slider to autoplay, but as my coding skills are very limited I am not sure how to, or even if it is possible to enable this on a Wix site. I have included the Apple description below.
Has anyone been able to turn on Autoplay for videos being used as backgrounds in sliders?
Information provided by wix is as follows:
Autoplay on Mobile and Tablet Devices
You may find that your music and videos do not play automatically on iPhones, Androids and tablets.This is because Apple and Chrome block the autoplay function to prevent additional data charges from the visitor's cellphone network. No data is loaded until the user taps the play button on the audio or video player.For more information, please see Apple's Safari Developer Library and Google's Developers guidelines.
Info from Apple Developer site:
Play a Video in Your Webpage
Using <video> elements in HTML5 markup allows you to embed video on your website.
Safari honors the preload="metadata" attribute, allowing <video> and <audio> elements to load enough media data to determine that media’s size, duration, and available tracks. A <video> element can use the play() method to automatically play without user gestures only when it contains no audio tracks or has its muted property set to true. If this video element gains an audio track or becomes unmuted without a user gesture, playback pauses. Playback stops if the video element isn't visible onscreen or is out of the viewport.Enable Video Autoplay
Video elements that include <video autoplay> play automatically when the video loads in Safari on macOS and iOS, only if those elements also include the playsinline attribute. For more information on inline playback, see Enable Inline Video Playback.
By default, autoplay executes only if the video doesn’t contain an audio track, or if the video element includes the muted attribute. Video playback pauses if the element gains an audio track, becomes unmuted without user interaction, or if the video is no longer onscreen (either by CSS or due to the user scrolling the page). The video doesn’t pause unless the user clicks or taps the video. This same expectation applies to WebRTC one-way video conferencing. For more information on WebRTC playback controls, see Add WebRTC Playback Controls.
Autoplay video when the user expects it to autoplay in your website. For example, autoplay video in interfaces where video playback is the purpose of the webpage, like a video-sharing or hosting website.