How to set baseline sound volume level before video editing & sound mixing
This article - How to Set Audio Levels for Video - offers some good advice. To summarize:


  Overall Mix Level: -10db to -14db
  
  Dialogue: -12db to -15db
  
  Music: -18db to -22db
  
  Sound Effects: -10db to -20db with occasional spikes up to -8db.


I usually set the voice a bit louder (-8 to -10 dB), but take these as a rough guide.