Unable to reproduce the issue on reported chrome version 67.0.3396.62 and latest chrome 69.0.3445.2 using Windows 10. Attaching Screencast for reference.

Steps 
--------
1. Launched Chrome.
2. Enabled screencsatify extension and disabled microphone.
3. Navigated to some video and Muted video also system volume.
4. Recorded video though screencastify.
Observed that the video rendering is not black and it rendered normally.

@Reporter: As we are unable to reproduce the issue from our end. Could you please review the attached screen-cast and confirm if anything being missed here and if possible could you please provide screencast for better triaging this issue.

Thanks! 

Deleted: 847715.webm 11.4 MB

	847715.webm 11.4 MB View Download

our implementation looked like this

background.js

navigator.webkitGetUserMedia({
    audio: {
        mandatory: {
            chromeMediaSource: 'system'
        }
    },
    video: {
        mandatory: {
            chromeMediaSource: 'desktop',
            chromeMediaSourceId: ACCESS_CODE
        }
    }
}, function (stream)
   {
        // always check the stream
        if (stream)
        {
            // get the html5 video dom element
            desktopvideo_dom = document.getElementById('screenvideo');

            // set this to muted so we dont double play audio
            desktopvideo_dom.muted = true;

            // generate the url stream
            desktopvideo_dom.src = window.URL.createObjectURL(stream);
        }


when desktopvideo_dom.muted = true; was commented out it worked just fine with the new update

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

@Reporter: As per your comment #4 (was commented out it worked just fine with the new update) could you please confirm can we close this issue?

Thanks.!

As there is no update from the reporter since a long time, marking this issue as WontFix.
Please feel free to raise a new bug if any issues are observed on the latest Chrome builds.

Thanks...