I doubt it would be any quieter, a bullet flying normally will slip through the air more efficiently than one with a large flat point which a bullet flying backwards would have.

Biggest advantage I know of for shooting a subsonic bullet backwards would be if it is a bullet that would not expand at subsonic velocity the larger flat point would cause more damage to flesh than a pointed one. Much like a flat point "meplate" bullet will rip more going through than ball ammo.