I assume if external noise is drowning out the music / changing how it sounds, then the music will be modified to let you hear certain sounds better via making them louder.
We use some techniques to rebalance your music - by making sounds that are softer in the mix louder, while keeping the very loud sounds at the same level. We also try to change the relative volume of the frequency bands you do not hear well, while preserving the overall volume.
Since psychoacoustics vary a bit from person to person, it may be perceived as mildly louder or softer than the original, but overall it will be at a similar volume level.