I found the following to work on iPad-Pro (iOS 13.4):
1. Change single instruments of an imported midi file
2. Adjust volume of a single instrument/track of an imported midi file
I imported a midi file from out of my files that I before exported as midi from out of the iOS app iRealPro into a files folder.
In iRealPro go to share - send audio - midi and select a folder to save the midi file.
It should work the same way with midi files from other sources.
In Garageband on iPad (iOS):
1. Change instrument:
- create a new song - select audio recorder as instrument
- on the top screen symbols menu on the left side change from instrument view to "track view"
- on the top screen symbols menu on the right side select the "loops" icon
- select "files" tab in the middle
- search/locate your previous exported midi file
- drag and drop the file icon of your midi file from the list into the tracks area
- Note: if you have exported from iRealPro, several tracks are created for each instrument
For each instrument you like to change, click on the very left side on the instrument symbol to select the track
- on the top screen symbols menu on the left side change from "track view" to single instrument view (small keyboard icon)
- in the upper part of the middle of the instrument view you can click on the instrument picture and select another from the library
- on the top screen symbols menu on the left side change from instrument view back to "track view"
voila, you should have the new selected instrument.
2. Adjust volume of one instrument:
To change the volume of a single midi instrument, that you can't using the volume slider, you have to do the following:
- in the track view click on the very left side on the instrument symbol
- in the upcoming context menu select "Automation" to expand the track to show volume details
- move/drag the yellow volume line up or down to adjust the volume of the complete track
- Note: to just adjust only part of the track you have to add marker point. To create points you need to slide the top left "pen" icon to the right first, tab on the volume line where to add a point and then push the slider back to the left move the line up or down.
Took me hours to figure this out!
Very very confusing, why apple made this all so complicated! :-(
Hope this helps!