Turns out that for the many things Macs are good for, getting computer audio to stream is a bit tricky due to some technical limitations (when compared to Windows). This means that if you want to share a movie or video with a friend, they might not hear the audio.
Disclaimer: this suggestion uses external software that we have no control over. Therefore, please proceed with the suggestion at your own risk.
Getting audio to work properly using Loopback Audio is very easy. However, Loopback Audio is fairly expensive. For the trial version, the sound will get a noise overlay after 20 minutes, which does not sound nice.
Step 1: Download Loopback Audio from: https://www.rogueamoeba.com/loopback/
Step 2: Install the software by running the installer you just downloaded. It should work out-of-the-box.
Step 3: With Loopback Audio installed, time to set it up. Initially you will see this view:
Step 4: Next to "Sources", click the plus button and select the app from which you want to stream the sound. We are using Chrome in this example.
We're watching the Old Town Road music video for the 57th time on YouTube, so you can see those green left/right channel bars indicate there is some sound coming from Chrome. A good sign, partner!
When you add an application, Loopback will automatically create routing to "Channels 1 & 2".
Step 5: Back in Kast, select Loopback audio for the "Input" in the sound options, shown in the image below:
Step 6: You are done! Enjoy your new found audio awesomeness
Phew, thanks for sticking through that, hopefully you should now be able to do what you actually want: share awesome content with your friends!