What you should never do is use the “-acodec vorbis” option of ffmpeg. Instead use “-acodec libvorbis”. This small change is very important. The former uses the the internal FFMPEG encoder, the latter uses the libVorbis library.

The internal ffmpeg vorbis encoder is about the simplest possible encoder that produces working output. It sounds terrible compared to the reference encoder. On a typical musical input the ffmpeg encoder set to 128k produces quality which is obviously worse than libvorbis at 32kbit/sec.