I'm kind of surprised you can hear anything outside your battlemech. With something designed to withstand the vacuum of space, drastic heat and cold, severe gravity and atmospheric pressure, etc... You would think there would be a rather big buffer between you and the outside world. You would hear your machine only and little else from the world around you (kind of like HPG Manifold all the time).
I guess you would still possibly hear footsteps as vibration from enemy mechs transfers through the ground into your mech through the legs. Then again.....
Wait this is all science fiction fantasy. Short answer, just because

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Wait, what were we talking about again?