Screen capture AND audio recording in ffmpeg
The solution seems to be to use both a different video and audio encoder. This line works:

FFmpeg/ffmpeg -video_size 1024x768 -framerate 25 -f x11grab -i :0.0+0,0 -f alsa -ac 2 -i pulse -acodec aac -strict experimental output.flv


I couldn't get it to work without -acodec aac -strict experimental, contrary to the HOWTO on ffmpeg wiki.

The error message I'm getting is:

Output #0, flv, to 'output.flv':
    Stream #0:0: Video: flv1 (flv), yuv420p, 1024x768, q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.34.100 flv
    Stream #0:1: Audio: adpcm_swf, 0 channels
    Metadata:
      encoder         : Lavc56.34.100 adpcm_swf
Stream mapping:
  Stream #0:0 -&gt; #0:0 (rawvideo (native) -&gt; flv1 (flv))
  Stream #1:0 -&gt; #0:1 (pcm_s16le (native) -&gt; adpcm_swf (native))
Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height


Neither could I get this to work using:

FFmpeg/ffmpeg -video_size 1024x768 -framerate 25 -f x11grab -i :0.0+0,0 -f alsa -ac 2 -i pulse -acodec copy output.mpeg


which contains -ac 2 probably in the right place. The error I'm getting repeatedly is:

[mp2 @ 0x7faffc007da0] Header missing

This most likely is a bug/regression.

To sum up, I've found a probable bug in both ffmpeg and on ffmpeg's wiki. And it looks like I'll have to buy the promised beer for myself ;)