In your code you tried to create MP4 container format output file instead AVI: when you've provided short name in av_guess_format as first parameter, it has more weight for decide output format than file name extension (https://www.ffmpeg.org/doxygen/0.6/libavformat_2utils_8c-source.html#l00198).

MP4 container does not support PCM data including G.711. Please look on this page for details https://en.wikipedia.org/wiki/Comparison_of_video_container_formats