Can you point me to the video you keep referring to?
also, the creator of the video said it's possible to get your own music onto lemmini
Sure, but the question still remains as to whether it's possible to get MP3 music onto lemmini, as opposed to MOD or other formats. It's possible that Lemmini did add support for MP3s later on and I just don't remember.
I'm assuming you aren't able to contact the creator of that video? Obviously if you can contact him/her, you should ask that person how it was done in the video. Also you might want to make sure you have the latest version of Lemmini, although I think if you downloaded it within the past year or so it's probably the latest version (I don't think it has been in active development for over a year or two at least.)
Sorry can't be of more help, I don't think you'll find many Lemmini users on this particular forum unfortunately. Maybe if you Google for Lemmini you might find other websites with more information. Good luck!