I have not had any issues with the voice chat as you describe, but then again I'm playing it on X-box 360, not pc...so yeah. Are you absolutely sure it isn't from some electronic feed back? I know when I'm doing something on the internet the sound through my headphones becomes distorted with static and other such noises. (The usb cable for my wireless adapter is right above the headphone jack so it causes interferance.) I do agree, an option to mute is needed, as is a toggle for voice chat in the case of PC users. (X-box has a switch for that)
As for sensitivity, well, again it might have to do with the fact I'm playing it on 360, but I have no problem aiming, even at the default setting (I've had people complaining at how good I was.) Then again, I'm usually predator so aiming isn't really that neccessary to begin with...
Considering the fact the game comes out in two weeks, I doubt there is much more testing to be done. It's already gone gold, so the project is pretty much finished. The demo however was based off of earlier code, so some of the issues might be fixed in the final version. But that is one thing about video games. It is impossible to catch everything, and all too often there is not enough budget set aside for propper testing.
Judging from my experinces in the demo, the game is pretty solid, and exceptionally balanced between the three races. On gamefaqs.com there are several threads complaining that alien is overpowered, or predator is overpowered, or marine is overpowered. (I do quite well with all three) If all three are overpowered, then it is balanced wouldn't you say?
Ultimately I look forward to the game, especially team modes on line. FFA is fun, but it just feels weird watching two aliens killing eachother...right before blowing their heads off with a well timed plasmacaster shot.