Others have authentic sounding instrument tracks. Files of this type won't be very good as BiaB songs. Some are little more than drum tracks with the vocal line played over it from a midi keyboard. The more popular songs have been copied into midi by many different people and in many different ways. The next most important thing is to find the best quality midi version of the song you are about to import. What makes the song identifiable? Is it the vocals, the bass, the lead guitar? You will need to know this in order to get the best sounding BiaB song you can.

Try to identify the style, number and type of instruments and especially any different parts. The first, and probably most important, thing to do before trying to import a midi file into BiaB is get to know the song as well as you possibly can.