I'm an iPhone7 Plus user and I too noticed that audio was awfully low from headphones all of a sudden and I tried EVERYTHING, and nothing seemed to work 😠😠. I now think I have found the solution as I just tried it out.
Navigate to Settings\Music and check under "playback". There are THREE settings there that definitely affect volume and headphone sound output.
There is "Volume Limit" which can affect the sound output level as does "Sound Check". When I set "Volume Limit" rather high, [and then re-adjusted the volume output of my phone] I heard music much better and [I wager] I will hear dialogue equally as loud when I get a phone call vs how it was before. Also I switched "Sound Check" off versus on and that seemed to increase the output versus lowering it. I guess that may be a limiter or a music "balance feature" of some sort. Each time I did have to make the adjustment again on the volume rocker on the side of the phone to increase the volume [as I did with the headphone volume rocker], but it was noticeably better than before so I think I have found the "smoking gun". I hope that this helps you as well. 🙂