stereo → mono stream Downmix both channels Mix a single stereo stream down to a mono stream. Both channels of the stereo stream will be downmixed into the stream: ffmpeg -i stereo.flac -ac 1 mono.flac Warning: Any out of phase stereo will cancel out. The following filtergraph can be used to bring out of phase stereo in phase prior to downmixing: -af "asplit[a],aphasemeter=video=0,ametadata=select: