There was some dirt in there, but it didn't help for me.
I have the iPhone 6+, while my wife still has a 5. I also have my old iPhone 5, which has no cell service of its own but can take calls as long as the 6+ is nearby. Both the 5 models have twice the volume of the 6+. I've tried all the ideas in this thread as well as other Apple discussion threads and on the web general and none have worked.
Interestingly, the volume used to be fine, but within the last week or so the volume crashed. Turning off Hearing Aid Mode (which some tips suggested turning ON) and noise cancellation helped some, but my phone is still barely half as loud as the 5 models I have.
Looks like I'm going to the Apple store. As this phone is about 6 months old, I suppose I can get a replacement. Assuming the problem isn't just inherent in the 6 models, which would pretty badly suck.