There may be different ways to do things ... a friend brought over a Midi File ( a tune with multi trks ) that he downloaded and needed help with. We could change the instruments as we wanted to , but the volume was set and couldn't be changed .... at least my me inna' hurry. Here's what I did.
I made a NEW trk for every instrument. A totally new blank track .... Then I copied each of the orginal trks to it's new trk. Put in the instrument I wanted to use on the new trk. After that I could control volume & pan as I wanted.
Then I deleted the orginal trks and saved what I had done.
So give this a try on just one track to see if it helps/works. Make a new track .... then copy one the others into that track .. you're coping the midi note info only. Then put the instrument want on that track and see if it works.