to work "as promised" a stereo-file is mandatory ! "Converting" a former mono to a stereo-file doesnŽt work as well !
So no oldies ( lets say anything recorder prior 1980, maybe 1970 ) recorded in mono
. YouTube is mostly as well not native stereo, so most files recorded from there wonŽt work either.
BTW: YouŽll, depending on the mp3 file being used, always hear "a bit" of the original-vocal ( in detail : that parts, that are not panned to center ). So e.g. "Little Talks" ( Of Monsters and Men ) wonŽt work, besause the man an woman sing on the left and right channel as far as I remember.