So you have Viber Desktop and want capture your conversation and stream it. Ok, no problem.
Step 1: Install SoundFlower.
SoundFlower is very popular open-source sound driver for macOs.
So download package from DOWNLOAD LINK and install it step-by-step. Please use right-click menu for open package (see picture)
Step 2: Setup Viber
Please open Viber (desktop of course) and open preferences dialog. Select "Soundflowe (2ch)" for "Audio Output"
Step 3: open JustBroadcaster and connect headphones to Mac
a) Add audio devices "SoundFlower (2ch)" and "Built-in Microphone"
b) connect headphones to Mac and turn-on icon "Headphone".
So you can call into Viber and start stream.
NOTES: for use Viber without JustBroadcaster please open Viber->Preferences and change "Audio Output" to "Built-in Output".