An example using the multitimbral kAudioUnitSubType_MIDISynth in Core Audio, and wrapped as an AVAudioUnit to use with AVFoundation's AVAudioEngine. The sampler av audio unit that is part of AVFoundation is not multitimbral.
You can build the project on the command line with this command: bundle exec fastlane ios test
I've not included a SoundFont to avoid any "legal" issues. You will need to find one and update the name in the source code. I've used SoundFonts from MuseScore.
If you find one, please add it to issues
Please read the LICENSE for details.