The volume on our 360 box varies quite widely between channels, and thus recordings, and also sometimes varies from one day to the next.
Previously the volume has been normally set to 12-13. However for the past couple of months some channels we need to adjust the volume down and for some, we have to raise the volume. For example BBC1 tonight we have had to raise the volume to 17.
It isn't our TV or sound bar, as the volume is stable when using TV+ and or Amazon 4K Firestick