En el manual he encontrado estas opciones, espero que os ayuden:
--audio-format FORMAT
Specify audio format: "best", "aac",
"vorbis", "mp3", "m4a", "opus", or "wav"; "best" by default
--audio-quality QUALITY
Specify ffmpeg/avconv audio quality, insert
a value between 0 (better) and 9 (worse) for VBR or a specific
bitrate
like 128K (default 5)
--recode-video FORMAT
Encode the video to another format if
necessary (currently supported: mp4|flv|ogg|webm|mkv|avi)
