The Monitor is how OBS drives a hardware device. "Monitor Off" keeps it silent. Change it to "Monitor and Output" for the sources that you want to hear.
Be careful though, that you don't also have a Desktop Capture or Output Capture source pointing to the same device that you use for the Monitor:
Because the captures come AFTER everything is mixed together, *including the Monitor*, using the same device in both places will produce an echo. The Desktop and Output Captures really do catch EVERYTHING that the specified device is doing.