I'm using ffmpeg 2.0
ffmpeg version 2.0 Copyright (c) 2000-2013 the FFmpeg developers
built on Jul 22 2013 19:48:32 with gcc 4.7 (SUSE Linux)
And I'm recording via these parameters.
ffmpeg -f alsa -ac 2 -i hw:0 -f x11grab -r 30 -s 1600x900 -i :0.0 -acodec pcm_s16le -vcodec libx264 -threads 0 output.avi
When I talk quietly, everything is ok, but when I talk normally or little bit loudly, in video is noise in background. I tried to record video on Windows with same microphone and everything was okay. Maybe I'm just running ffmpeg with bad audio codec, but I don't know.

Does anybody know how to fix it?

Here's the video, it could be available after 45 minutes of posting this thread.