Agreed, it's really loud if you don't have noise isolating earphones. Reducing the noise level by a bit might reduce stress levels in passengers, but probably not by much.
Well the external benefit could be much more: more people would be more likely to take mrt, less noise pollution for neighbourhoods - which is the exact problem they are facing now.
It was one of the many cost cutting measurements made during the purchase of rolling material but also the tracks.
I don't know why people downvotes this comment, but they are violating reddit etiquette. Yes it is definitely plausible since there are times when the government wanted to save money for other stuff.