Ok, I just received my 6p last night.  We already know that it's a great phone overall; so after going through the initial setup, the very first thing that I tested was music through the headphones.  First of all, I had no problem with the headphone jack - both my Bose in-ear headphone, and my Bose over-ear headphones slid into the jack perfectly and easily.  But I'm sorry to report that the sound quality and volume is woefully lacking.  I'm a bit of an audiophile, but think that most anyone would be completely uninspired by the sound quality.  Maximum volume is what I consider to be barely at comfortable listening volume in a quiet room.  In a loud environment (airplane), the volume will be insufficient, even at max.  And the overall quality was a big step down from my Sony Z1S.
I plan to keep the phone, for all of the other reasons that we're aware of.  But I have to say that I'm pretty disappointed that a flagship phone would overlook audio quality to this extent.  It's very unfortunate.
Anyway, I just wanted to report my findings, so that others could add this to their decision-making criteria when picking out a new phone.  If you are an audiophile, and music is one of your top criteria for a new phone, you will most likely be disappointed with the 6p.
Question:  Is there any chance that a future software upgrade could improve the audio quality?  Or is that a function of hardware only?
Thanks.