When monitoring mono audio via BlackMagic devices I have previously had to change the Audio Midi Settings so that Multichannel Audio is set to Stereo. If you don't do that you don't hear any sound out of the BlackMagic outputs 1 and 2.
That's worked fine up until now, where I have a clean install of macOS Sierra. Here there does not appear to be any Multichannel option in the Audio Midi settings, and therefore I can't hear any sound.
Interestingly, if you have changed these setting in a previous OS version, then upgraded to Sierra, all is well - it works! You can't access Multichannel audio, but the settings have stuck.