There are two ways to do this: disabling auto-playing media, or just muting the video.
To disable auto-playing ads, follow the following steps:
1: go to your user settings page on Reddit.
2: click on the "Feed Settings" tab.
3: toggle off "Autoplay media. Play videos and gifs automatically when in the viewport." Having done this, the ads with sound will stop playing.
To mute the videos, click on the speakers icon at the bottom right of the video play. Reddit will remember whether or not you have videos muted, so all videos will become muted if you click on this.
Observations
There are a few things that could be considered "media" as far as Reddit is concerned, here's how this setting affects each of them:
Videos with audio hosted on Reddit: Autoplay Media works as expected: if it's on, it plays, if it's off, it doesn't play, and you need to manually click play. I tested this on /r/RocketLeague, which often has gameplay clips with audio being posted.
GIFs: they always play, even when Autoplay Media is disabled, so if you're a /r/HighQualityGifs subscriber, you won't miss anything.
Videos hosted on Youtube: they never play automatically, even if the setting is enabled.
On top of that, it's worth noting that if you mute a video that has audio, other videos that have audio will also be muted. This means ALL other videos, both promoted videos and normal video posts.
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