I don't believe it is possible to adjust the volume on a MIDI track in the interactive music hierarchy. That can only be done on the SFX Object that the MIDI track is targeting in the Actor Mixer Hierarchy. You can set that to respond to pre-recorded CC volume information on the MIDI track by setting up an RTPC in the SynthOne source editor on in the Sound Porperty Editor of the SFX object itself:
https://photos.google.com/share/AF1QipPC7cAnxDmXCJXj6fiwOeYdd4s1SdszV9zosy-dx09BIhYGO-t1WaRUeAoTH73voQ/photo/AF1QipM_RDxT1yV9RK6d0puCL6L31qUvXW2dWl4SL095?key=WTZvVXpvNnBzc1NLZnJUN3psY2p6LXBvNENjWjJR
Either of those will adjust the volume according to incoming CC7 (MIDI Volume) messages.
You can monitor the volume of your synths by creating a separate audio bus as a child of the Master Audio and targeting that bus in the Sound Property Editor of the synth.
Not sure if that is what you're looking for, but basically any volume adjustments to a MIDI track have to be done to the SFX object in the Actor-Mixer hierarchy that it is targeting.
https://www.audiokinetic.com/library/edge/?source=Help&id=mixing_midi_and_audio_contents