In alsamixer hit f4 to go to the capture screen, scroll to the volume bar for mic/front mic or whatever your input device is and hit spacebar. That should put a red L R under the channel and make it active so your mic signals get through. (Adjust volume as necessary)
The screen you showed appeared to be the output screen, if you unmute your mic there you'll just hear everything you say through your headset. When you're done, press esc to exit and then type "sudo alsactl store" to make it persistent.
Just out of interest, is this 32 or 64 bit and what processor and gpu do you have? Heh damn arch-games people being lazy again; that isn't an official project as far as I know and last time I checked, it doesn't get updated very often.
As for your issues with the actual game, I haven't built it in quite a while, when I get home tonight I'll try building with the latest PKGBUILD and see what I get. My last build is dated 5th June, and the md5sums in the new PKGBUILD match the old downloads I have for the actual game/engine/whatever. Did you ever run UrT in Ubuntu? As in, is this a problem with Arch specifically? I know some people require a special loader for Ubuntu for... something, I can't remember what it is now.
EDIT: I've just built and tested from the most recent PKGBUILD, no noticeable difference in FPS on my end. (I only did a quick spec just to see what the values were on the same map on the same server)
When running under Windows, did you tweak any of your settings for performance? I'm sure you've checked all this kinda stuff but I'm grasping at straws a bit here.
System specs might be helpful, and also what DE/WM are you running? I know a secondary X is supposed to background the other session, but it might be worth trying a lighter system and see if that makes a difference.
EDIT2: My train of thought is running away with me here... GPU brand is important, please tell me it's nVidia and not ATi or Intel?!