Yes it's burnt into the audio track they are playing.
Airline mixes are commonly used in economy class IFE. BA is no exception.
Most people are interested in being able to follow the film and if they didn't you would lose low volume dialog underneath the cabin noise.
Try flying J or F where they may not do this so much (quieter cabin, better headphones).
Or as many people do, use your own devices and ignore the provided IFE completely.