Well, I have been a long time user of Multitracker. I bought ST3, but it seemed best for a solo, duo or trio setup but not a full band. So I continued with Multitracker until I saw that ST4 has all of the features for which I was looking (that wasn't there before). How could I say, "No?" So I upgraded.

All my files are m4a. A few issues I am having.

My audio interface is UMC1821 by Behringer. Works perfectly with Multitracker. I installed ST4, hooked up my audio interface, routed 7 (2 stereo and 3 mono) of the 16 outputs and disabled the other 9. Loved the interface, loved the routing flexibility, created a few songs.
I set all of the settings of which I could think.

No audio playback. I power cycled everything. Noted that ST4 detected my connecting and disconnecting the audio interface. Still no playback.

I have worked with iPad audio for several years and used backing tracks for a little under 600 shows over 12 years.

I thought that maybe I should just go back to Multitracker for a minute until I increase my knowledge about ST4 (which I plan to use instead of Multitracker on my iPad or Ableton Live 12 On my PC).

Only problem is that now, regarding Multitracker, the use of ST4 has made my UMC no longer detectible by Multitracker on my iPad. I resolved this and restored functionality of UMC with Multitracker by resetting all iPad settings.

ST4's handling of external audio interfaces has me deeply concerned! It removes audio interface functionality for other iPad multitrack apps! It can only be restored with an iPad reset. (same issue on 2 separate iPad Pros)

Two problems.

All routing set right, levels set properly, songs created, but no sound (maybe m4a isn't supported?)
ST4 knocks out audio interface detectability by other iPad audio apps unless the iPad is reset.

Any help would be great. I want ST4 to be my new track handler because it has the mixer interface and MIDI file support! At this point it is super risky because of its effect on my established performance app! I have an iPad11 with the M2 processor as my test environment so far. I was able to regain/reset my iPad pro 12.9 with the M1 processor, so I can continue normal functioning with Multitracker.

Any help is appreciated, but the basic troubleshooting has been exhausted. I am very advanced at working with this type of technology. ST4 has me stumped. What am I missing?

John