You are correct that there is not a central dedicated Master Volume knob as of now. However, most ppl use the two Gain knobs for each Deck to control overall volume (in addition to the level faders).
As for sync, when you press the Sync button Traktor should attempt to Sync the current Deck (based on the Sync button you press) to whichever Deck is currently the "Master" (you will be able to tell which Deck this is by seeing the Master indicator in Traktor light up yellow).
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