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)?

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    $\begingroup$ Mosquitoes can fly and that is a greatly advantageous. The buzz is a tradeoff for their flying ability. I don't understand what exactly are you getting at. $\endgroup$ – WYSIWYG Jun 17 '15 at 5:01
  • $\begingroup$ @WYSIWYG: well, as a colloquial example, gliders, airplanes and choppers both fly, but do not make an equal amount of noise, do they? My point is - why can't the noise be either less intense, or outside the audio range for humans. (Thanks anyways, maybe I'll add this to the question.) :) $\endgroup$ – 299792458 Jun 17 '15 at 5:05
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    $\begingroup$ Maybe because most mammals are four-legged, thus they can seldom protect themselves against bites. Even though a cow might hear a mosquito land on it's back, can't do much about it. Humans on the other hand, are only here for as little as 200,000 years, which is not enough for much changes to occur. $\endgroup$ – FloriOn Jun 17 '15 at 10:48
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    $\begingroup$ This question is clear and there is no reason to close it even though the main answer might be pretty straightforward. $\endgroup$ – Remi.b Jun 17 '15 at 15:35
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    $\begingroup$ this is a frequent sort of question that arises on the site - why doesn't evolution eliminate obvious weaknesses. $\endgroup$ – shigeta Jun 20 '15 at 16:27

Your question is really good, but in actuality, they do evolve towards muting themselves, actually, they have pretty much done "most of the work":

It is assumed that microscopic scales along veins and wind margin
play an important role as a silencer as downs on the flight feather of owl.

From this we can assume that the effectiveness of these scales are high, as they had plenty of time to evolve; thus the other factor they should alter to make them quieter is the frequency of their flapping $f$, which can be calculated from the following equation:

$$f=K m^{-1/6}$$

where $m$ is the mass of the animal and $K$ is the proportionality constant. The resulting number is around several hundred hertz (Remi.b observed a value around 440 Hz)

There are two options for them, either increasing the frequency up until our ear is incapable of hearing it which would be impossible as even the highest flapping frequency can hardly exceed 1000 Hz, which is nowhere near the required 20,000 Hz. By increasing their body mass they could decrease the frequency but the needed 15-20 Hz is pretty low compared to the actual several hundred, thus it would likely need a complete body structure change which is not something to happen in the near-future.

An another solution might be a different strategy in flying. Gliding attacks might pay off really well (that's why owls on the other hand can't be heard during flight), but even though there are some insects capable of such[1], it would need an immerse amount of time, not talking about the problems that might arise from the bumping into of our body.

I don't think that the mating signaling would be indispensable from an evolutionary perspective, but this statement of mine can't be proved.

These are the physical/biological difficulties of the problem, here is one from an evolutionary aspect: They are not likely to have experienced a strong selection against this feature, as most of the possible victims are incapable of doing much against a mosquito due to the absent of hands (and tails are seldom enough to stop them). The first possibly blood-sucking mosquito was found to be living 79 million years ago, while animals that might be able to "seal their fate" are much more recent, while not even having success most of the time.

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    $\begingroup$ Thanks, this is a very well researched answer. I'll wait for sometime and hear from others if they have anything to add to this, before accepting. $\endgroup$ – 299792458 Jun 20 '15 at 14:34
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    $\begingroup$ ..they have not experienced strong enough selection to be silent fliers. people can only really hear those things when they fly close to your ears. $\endgroup$ – shigeta Jun 20 '15 at 16:28

Gliders do not actively fly and there is no work done by them in producing motion. Other flying machines do make a sound.

If a trait exists then it means that there is not enough pressure against it. You can hear a mosquito only if it flies close to your ears; otherwise, they seem to be undetectable by hearing.

To produce ultrasonic sounds the mosquito has to flap its wings too fast- which is energy intensive. Conversely, to produce just infrasonics, it has to flap the wings too slowly which may not provide it with enough thrust.

A recent study by Bomphrey et al. (2017) suggests that mosquito flight is different from other insects and the aerodynamics are rather complex. Mosquitoes are known to use low amplitude, high-frequency flapping that generates the buzz.

From the abstract:

Mosquitoes exhibit unusual wing kinematics; their long, slender wings flap at remarkably high frequencies for their size (>800 Hz)and with lower stroke amplitudes than any other insect group1. This shifts weight support away from the translation-dominated, aerodynamic mechanisms used by most insects2, as well as by helicopters and aeroplanes, towards poorly understood rotational mechanisms that occur when pitching at the end of each half-stroke. Here we report free-flight mosquito wing kinematics, solve the full Navier–Stokes equations using computational fluid dynamics with overset grids, and validate our results with in vivo flow measurements. We show that, although mosquitoes use familiar separated flow patterns, much of the aerodynamic force that supports their weight is generated in a manner unlike any previously described for a flying animal. There are three key features: leading-edge vortices (a well-known mechanism that appears to be almost ubiquitous in insect flight), trailing-edge vortices caused by a form of wake capture at stroke reversal, and rotational drag. The two new elements are largely independent of the wing velocity, instead relying on rapid changes in the pitch angle (wing rotation) at the end of each half-stroke, and they are therefore relatively immune to the shallow flapping amplitude. Moreover, these mechanisms are particularly well suited to high aspect ratio mosquito wings.

Bottomline is that evolution does not know where the best optimum is and it does not really try to proceed to a global optimum. If a trait is disadvantageous, it is selected against. Neutral mutations stay but do not get amplified unless they have some kind of advantage.

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    $\begingroup$ Note that the speed of wing flapping is exactly equal to the frequency of the sound produce. A mosquito flap at about 440 Hz (personal observation during my Bachelor degree). A young and healthy human can hear up to 20000 Hz. The mosquito would need to flap their wings 45 times faster to become inaudible (and would need to adapt its mating behaviour in consequences). $\endgroup$ – Remi.b Jun 17 '15 at 15:37
  • $\begingroup$ @Remi.b - Wow. That's fabulous information! Any comments about the second proposal (lesser intensity of sound, even though at the same frequency)? $\endgroup$ – 299792458 Jun 17 '15 at 15:50
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    $\begingroup$ @TheDarkSide for exactly the same reason as explained in the answer - there is no selection pressure to lower the volume. Additionally, since mosquitoes find each other by sound, lowering the intensity would decrease numbers of mating attempts, something that I'm fairly sure would not be advantageous to the species. $\endgroup$ – MattDMo Jun 17 '15 at 23:23

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