Upload your video somewhere on the internet (Youtube, Google Drive, Powerpoint online)
Share your screen in Juno.
Have whichever website your video is uploaded to open in a tab in the chrome browser.
Click to share a chrome tab and select the tab with your video.
Click the share audio box in the lower left hand corner.
NOTE: if you share a video through the powerpoint app or by sharing your entire desktop, you will not be able to share the audio, so please be sure to share your screen by sharing a chrome tab.
