Bernie,

If you are using prerecorded midis from the internet, you will need to rearrange the tracks on the midi so tracks one and two handle the melody and track 3 handles the bass. If there is no melody for track 2, leave this track out of the midi so your tracks will run: 1,3,4,etc. This can be done with midi software. Then when you load the midi into the keyboard, just mute tracks one, two, and three. What is happening with the volume changes is that the midi tracks are being told what to do inside of the midi program. If you mute the tracks, the midi program will not control them. I hope this helps. If it's not clear, just post again.

Peter

PS. Midis from the internet come all messed up and need to be remixed before using on the keyboard.