ffmpeg 音频转换: use ffmpeg convert the audio from stereo to mono without changing the video part

To convert the audio from stereo to mono without changing the video part, you can use FFmpeg:

ffmpeg -i input.avi -c:v copy -c:a libmp3lame -ac 1 -q:a 2 output.avi

The important option is -ac 1, which downmixes the signal to one channel. Note that this will re-encode the audio, so expect some quality loss.

To change the quality for MP3, choose a different value (from 0 to 9), where lower means better. 2 corresponds to around 95 kBit/s per channel IIRC.

In Ubuntu, an old version of FFmpeg is probably supplied in the packages. Don't do apt-get install ffmpeg and rather download a static build from the download page, or use Libav, which provides the avconv command. It's a fork of FFmpeg with similar functionality and usage and available in the Ubuntu packages.