I recently need to play my computer sound to my audience through youtube live.
By default, on Mac OS, you can only select build-in microphone as the audio input.
Then I found this solution.
Download and install Soundflower.
Then run Audio MIDI setup, create a new multi-output device.
Now go to system sound preference, set the multi-output device as output.
Then set this multi-output device as google handout's input. You are done.
But, you will notice that you cannot tune the volume any more.
You have to use these two slider to adjust the volume.
But last thing, you computer sound will be played to your audience, but your own voice will not. Your real micphone doesn't send audio through google handout.
Here is a workaround, use this lineIn app to redirect your microphone to multi-output device. You are all good to go. You can mute your mic using it too.
download from https://www.macupdate.com/app/mac/11333/linein
