This issue is impacting me too. For hours today (and many many times previously) the music plays at a reduced volume. Tried all the fixes mentioned, attention aware, notification volume, restarting the phone, reinstalling softwareisolating the issue to one app.
Today, the app that caused the volume to return to normal level was launching the camera app.
Furthermore, the answer can't always be to restart the phone or reinstall software. Sometimes we are running apps that can't be interrupted with a restart and we certainly shouldn't have to reinstall software every time any thing is wrong with the phone (seems to be Apple's equivalent to ibuprofen). Nevertheless, neither fixes the issue.
Another related issue, my music randomly stops playing when listening via wired headphones. Though it was the attention aware or lift to turn on, so disabled both. Unfortunately, the problem continues. For example it's happened while walking with phone in hand and phone in pocket. Sometimes happening multiple times in a row. I thought maybe the 1/8" plug was partially disconnecting resulting in in stopping, but with testing I couldn’t reproduce it when partially pulled out.
All I want is to listen to music without fighting my device.