Quite certainly, muted mosquitoes would be much more effective as far as their blood-sucking pursuits are concerned, since mosquito sound is predominantly responsible for sealing their fate (between the two palms of the hand). Muting themselves would certainly reduce the chances of being caught in the act. For instance, unless you notice by looking, leeches go undetected for long periods because there isn't any obvious sound emanating from them.
Thus, reasoning says - this should be favorable from the point of view of evolution, unless there is a some indispensable purpose served by this sound, which can not be otherwise served. This page seems to suggest that this is so from the point of view of mating. In fact, quoting verbatim:
Since female mosquitoes are larger, they flap their wings slower, and males know it. They use the distinctive pitch of the females' buzz to recognize them. Louis M. Roth, who studied yellow fever mosquitoes for the U.S. Army during World War II, noticed that males ignored females whenever the females were quietly resting, but whenever the females were flying, and therefore buzzing, the males wanted to mate with them. The males even wanted to mate with recordings of female mosquitoes or tuning forks that vibrated at the same pitch.
But mating signals could also be of other forms, like some chemicals secreted (I envisage something like pheremones). Why is making sound so important? Why can't this noise be either less intense, or lie outside the audio range for humans (their targets)?