If you can record your voice and play it back fine, it may still be that the output volume of the mic is not high enough. If you are playing your recorded voice back on your Windows desktop it would be easier to hear as there are no background game noises. If you have the mic output dialled all the way up, I know in Vista at least (and probably XP and Win7 as well) there is a little checkbox you can tick to give it an extra boost. I've had complaints that I'm too quiet before as well and it’s only by checking that box was I able to bring my mic level up high enough. In Vista you should find the box under the sound control panel and then click on the recording devices tab. Select your microphone then check the box. It's something like that anyway. It should be obvious when you see it.
If you wanted to test it out in game, I don't play ARMA II either, but I know in TF2 (and most other MP Source games I believe) there is a little microphone test button. Basically what that will do is play your voice back but with a second or so delay so it is easier to hear. There may also be something in ARMA II, I'm not sure. Also make sure all the obvious options are checked in game as well.
If none of that works, it might be you have a duff mic (although it sounds new from what you suggest in your post, but some stuff is broken out of the box). Try wiggling the wire going in to the mic and any other place which looks like a weak point for the cable to see if you can 'fix' it, albeit probably temporary. You would want to be constantly speaking in to it while recording or something as you are doing this to test. Try and see if you can feel breaks in the wire as well and try wiggling that about a bit to see if you can make it come good.
