In the mixer, there is a setting for main volume on one of the dials that lay between the channels and the "AUX/MAIN" section of the mixer. One is called 'All'; make sure it is all the way up.
There is also a slider in the same Mixer window, on the bottom right that is the main output volume to the soundcard. This is usually labeled A1, and is the main audio output level; make sure it is also all the way up.

Realband routes all the outputs from the mixer to the 'All' column first (which is a master volume output control) then what goes thru that goes to the stuff on the right side including the main output from the mixer (the A1 slider on bottom right, but not all the way right)

These could both effect master output volume.
BTW, that A1 output has an FX area and this is where you would want to put any mastering effects so they affect everything leaving RB.

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