If the previews aren't playing back on hover in Chrome, you need to change the current autoplay settings in the following way:

1. Launch Chrome
2. Go to chrome://flags/#autoplay-policy
3. In the 'Autoplay policy' settings choose ‘No user gesture is required.’
4. Click on the 'Relaunch now' button.

If the previews aren't playing on hover in Safari, you need to change the current autoplay settings in the following way:

1. Launch Safari
2. Go to the website you want to allow to autoplay media
3. Go to Safari/Preferences/Websites/Auto-Play and choose the 'Allow All Auto-Play' option