Re-enable mono, it works fine. Example command line:

18:36 antoine@macbook ~/videolan/vlc-build% ./vlc ~/media/music/Greenday/American\ Idiot/01\ -\ American\ Idiot.mp3 --sout '#transcode{acodec=mpga,ab=32,channels=1}:std{access=file,mux=raw,dst=/tmp/a.mp3}' -vvv -I dummy --no-media-library

(with pre 9107e1e0 builds you need to add --sout-mono-downmix to the command line)

NEWS

@@ -50,7 +50,6 @@ Important notes:
    transcode{vcodec=mp2v,vfilter=canvas{width=640,height=480}}
  * Glide video output module has been removed.
  * Due to lack of maintainer, the Skins interface might not work properly.
- * Transcoding multiple audio channels to mono does not work anymore.
 
 Changes:
 --------

configure.ac

@@ -1160,7 +1160,7 @@ VLC_ADD_PLUGIN([nsc])
 VLC_ADD_PLUGIN([xtag])
 dnl Channel mixing audio filters:
 VLC_ADD_PLUGIN([converter_fixed])
-dnl VLC_ADD_PLUGIN([mono])
+VLC_ADD_PLUGIN([mono])
 VLC_ADD_PLUGIN([trivial_channel_mixer])
 dnl Resampling audio filters:
 VLC_ADD_PLUGIN([trivial_resampler])