I just did the Marshmallow update a couple of days ago, and haven't noticed any difference in volume. Calls seem to be about the same. I listened to music on headphones (PSB M4U2) from Google Play Music on 3 flights this week since the update, and audio seems the same for that setup. I used my bluetooth speakerphone (Jabra 510) with the DT2 updated to Marshmallow for several conference calls and THAT sounds the same. I finally just pulled up a Youtube video (Anna Nalick, Breathe / 2AM) and listened just on the phone speakers - and it's nice, loud and clear.
Edit: also used for numerous calls since the update - all seemed normal.
I wonder what's causing the intermittent results you're experiencing?