Replaces Tumblr's video player (that lacks volume control) with a player with a volume bar.
Find the source code at https://github.com/jtank4/Better-Tumblr-Video
Replaces Tumblr's video player (that lacks volume control) with the Plyr (https://plyr.io/) video player (with volume control) or Chrome's video player (user's choice).
Select your video player by left clicking on the Better-Tumblr-Video icon in the top right corner of Chrome.
Caveats: Doesn't work on all pages. Some pages of tumblr, like the search page and certain user pages (depends on theme), are completely ensconced in an iframe, which prevents the extension from seeing any videos on the page.
Needs permissions: Storage - to store your preferred default video volume.
Change log: https://github.com/jtank4/Better-Tumblr-Video/commits/master