Major Tom said:
It depends what you are using the sound suppressor for. If you are going for as silent as you can get, then yes, you can use subsonic ammunition.
Largely in CoD when you're given a silenced weapon it's for a "silent" mission, so it would be understandable; the sound used is also a lot more consistant with silencer + subsonic than with full-power rounds. Could also just be our avatar is worried about wear on the can if he starts throwing supersonic ammo through it on a regular basis.
Also, since you're pointedly using "suppressor" instead of "silencer;" while that's what gun enthusiasts tend to do, it's not right that "silencer" is an incorrect term; Maxim patented his design as "Maxim silencer."
Major Tom said:
But for general military use standard ammunition is fine, 9mm para rounds tend to give good subsonic performance out of the factory anyway.
Yeah, but 9mm para isn't a rifle round. The SCAR-H is a battle rifle that fires a full-size 7.62x51mm NATO, try firing that supersonic out of a silencer and still being in any way quiet.
Major Tom said:
Suppressors are used mainly to reduce muzzle flash diffuse the sound of the discharge, so that it is difficult to determine where the shot came from, not to make the weapon silent.
Actually they're used
mainly by police and antiterrorist units who are either worried about decibel reduction for health and safety reasons, or want teams to be able to use verbal signals indoors without deafening each other every time they fire.
Snipers mainly use them for the reason you state, but noise reduction is the most common usage. Of course a silencer doesn't ever make a gun particularly silent unless the weapon is built around it [as per the Welrod, De Lisle and VSS, among others], but hey.