These steps assume you have at least Shush 1.2.1 installed.
1. Quit Shush.
2. Launch the Teriminal application.
3. Paste the following command, as a single line, then press Enter.
defaults write -app Shush blacklist -array-add "com_globaldelight_driver_Boom2Device:Boom2Device"
4. Restart Shush.
Shush should no longer mute the Boom 2 audio device, but if you still can't hear any audio, then your Boom device is currently muted.