Assuming the same range. a sub-sonic round has to be heavier to do the same damage as a faster round.
Another problem is a sub sonic round must have a higher "arc" in its trajectory compared to a faster round , which requires more precise estimation of range (If range is wrong, greater chance that the round will go over or under the target).
One problem with heavier rounds, is they take more powder to to pushed out of the barrel. . Thus you have to not only silence the round, you have to silence the propellant. Now, Silencers have done this, but silencers are noted for not lasting long, mostly due to water build up (the technical difference between silencers and your car muffler is the size, mufflers last a long time for they are silencing much lower exhaust gas speeds BUT tend to be 100-1000 times larger then most silencers).
The "Silent" weapon of choice, when people wanted to target something outside of pistol range (about 20-50 feet) are the following, for it fires a very heavy projectile over a high arc, but with a propulsion system that stays below sonic speeds, It is reusable and thus does not have the problem of filling up with water like a conventional silencer:
Another silent weapon, with more power (and thus greater weight AND greater ability to penetrate Armor), but slower rate of re-loading:
http://image.shutterstock.com/display_pic_with_logo/629497/629497,1281976555,3/stock-photo-medieval-girl-shoots-a-crossbow-59143561.jpg
http://image.shutterstock.com/display_pic_with_logo/629497/629497,1281976555,3/stock-photo-medieval-girl-shoots-a-crossbow-59143561.jpg
The British issued bows during WWII to some units, through except for one report in 1940, none seems to have actually been used in combat:
http://www.warhistoryonline.com/featured-article/glorious-bastard-mad-jack-churchill-became-the-only-man-during-wwii-to-kill-an-enemy-soldier-with-an-arrow-fired-from-a-longbow.html
Please note, most unconventional operations would NOT mention how someone was "eliminated" but combat reports would. I have seen photos of British Troops, all with rifles then one with a Longbow. This may just have been a propaganda photo, but it exists and longbows were obtained and at least one was used (I have also saw a cover of Life Magazine of 1944, of US Service men marching in formation, all with M1903 Springfields. I suspect that was an attempt to fool the Germans that most US Troops still had Springfields instead of the M1 Rifle. You have to be careful when you review photos published in war time, for fooling someone may be part of the reason for the photo).
In Vietnam, silenced pistols and sub-machine guns were used, but the US also had its Hmong allies use their cross bows when silence was wanted. Again, in open combat the Hmong preferred modern weapons, but I have heard of reports where the Hmong used Crossbows to take out men on guard duty for any Viet Cong or North Vietnamese group they were attacking.
Just pointing out, such weapons have a place even today and fulfills the "requirement" of a silenced weapon.
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