[in menu Modify/Volume operations]
This operation removes the notes from melody channel. That is useful for karaoke purpose or to play the melody self on a MIDI keyboard along to the remaining song accompaniment.
First you need to define the MIDI melody channel of the song once in the MIDI descriptions of the file. This operation automatically starts the description dialog if the melody channel is not set.
The description dialog offers also to define a transpose value for the MIDI file. This is useful when your score sheet contains the notes in other song key and the notes (except drums) will be automatically transposed to same song key when applying this operation.
This operation can be tried also for a Mp3 file if FFmpeg package is installed.
In fact it only removes audio signals at center stereo pan assuming that many songs use vocals in stereo center and other instruments at left or right pan.
Hint: This surely does not work for all songs and the quality of resulting audio file might be reduced.
Currently there is no better approach for removing vocals available in FFmpeg (and probably other tools).