I have been using the aux. input for midifiles from an external roland midifile player/sound

I use line in, you have a volume control on the keyboard giving you an easy way to balance.

1-Main volume control
2-Control for midi files (line in)
3-Control for mic. if you user one.

Technicsplayer had posted some good information about the Aux impute about a month or so ago.

John C.