This is what I found out.
You select all your tracks in the music track, go to Audio Mixing room (F9).
There is a right bar for adjustng Music. I noticed that if you drag vertical bar you only andjust the volum of the first music track, however if you drag the horizontal small bar below, it adjusts audio level for the rest of the music tracks.
Read not to contradict and confute, nor to believe and take for granted... but to weigh and consider.