Следующая команда ffmpeg приводит к ошибке. Когда добавляется «-c: копия», чтобы просто скопировать аудиопоток, видеопоток красиво перекодируется с запрошенным битрейтом.
Codec is experimental but experimental codecs are not enabled, try -strict -2
When converting to MP4, ffmpeg tries to choose an AAC audio encoder. Since your ffmpeg is not compiled with libfaac, libfdk-aac or libvo-aacenc, all of which are third party encoders, it defaults to the internal AAC encoder (called aac).
However, this encoder is considered experimental and will only run when you add -strict -2 or -strict experimental to the options.
Some notes:
You are trying 10 kBit/s as audio bitrate which is much too low for the built-in AAC-LC encoder. This bitrate would only work for HE-AAC v2, which libfdk-aac could do. And it'd still sound bad.
In general, I would always explicitly specify the encoder you want to use. Saves you a bit of trouble when the defaults are not sane.
I'd recommend you update your ffmpeg version to something more recent, either by downloading a static build, or compiling it yourself. When you compile, you have the choice of using libfdk-aac, which offers better quality than aac, and also has a proper VBR option.