Hi, same problem with my iPhone 7s post the 11.0.3 update.
Here’s a little more detail and a temporary manual workaround:
1. When I start playing music or podcast the volume plays at about 50% of what it should be.
2. Same experience from headphones, Bluetooth, and the phone’s own speaker.
3. While continuing to play in the background, launch the camera app (or Shazam app works too) then close that app down (swiping it up after a double click on the home button) and the sound comes back to 100%.
4. I havent found any other apps that have the same effect.
5. Pausing the music or forwarding a track brings the volume back down to 50%.
hoping they fix this soon!