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.
But don't fear, we've got your back with a few suggestions!
Disclaimer: both of these alternatives use external software that we have no control over. Therefore, please proceed with these suggestions at your own risk.
Method 1: Soundflower
Soundflower is a piece of technology developed for Mac many years ago, but the latest release (from 2014) still seems to do the trick. This method is a bit tricky, but Soundflower is a free alternative.
Step 1: Download the "Soundflower-2.0b2.dmg" installer from: https://github.com/mattingalls/Soundflower/releases
Step 2: Follow the installation instructions on that page (https://github.com/mattingalls/Soundflower/releases). Notice that getting the installation to work will require you to "accept" the application in the security settings, then running the Soundflower installer again.
Step 3: Great, now you have Soundflower installed on your system. This will work as a virtual audio devices, as we wizards in the industry call it.
Step 4: Open up the "Audio MIDI Setup" utility which is include in macOS. It should look something like this:
Note: depending on what type of audio devices you have installed, your view might be slightly different. The important thing is that you can see the Soundflower (2ch) in the left side bar.
Step 5: In order for your viewers to hear audio, just select Soundflower (2ch), click the cog wheel at the bottom and choose "Use this device for sound output".
Step 6: If you after this can't hear the audio yourself, try the following:
In the Audio MIDI Setup, click the "+" icon on the lower left corner, and select "Create Multi-Output Device", and give it some cool name you can remember (we went with Kast Megamixer3000). Select this, and make sure both "Soundflower (2ch)" and "MacBook Pro Speakers" (or whatever computer model you have) checkboxes are checked under "use". See the image below for clarification.
Finally, do the same as before: with this selected, click the gear icon and choose "use this device for sound output". Now the sound should be directed to both you and your viewers.
Note: when using Soundflower, in the Kast app audio settings (the gear icon when you are kasting) it should be enough to have "Default device" selected as your output. If this does not work, try selecting "Soundflower (2ch)" in the Kast audio options.
Method 2: Loopback Audio
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!