I use Google Hangouts all the time. I love the service but sometimes the audio doesn’t play nice on Macs. The solution can be as simple as changing the settings in Google Hangouts and System Preferences -> Sound. Today Hangout audio failed and this didn’t work. Doing a little searching, this command and re-joining the hangout worked for me.
sudo killall coreaudiod
I’m keeping a note here so I can grab this command next time it happens.