Thanks for posting this Chris. And I'm glad to hear it sounds like you have a backup plan for your show.
For everyone here, I just wanted to chime in and say that we are aware of this issue and tracking it very closely. Since the betas are unfinished software, we don't have any answers yet. But I can tell you what we know so far.
When running on iOS 17 beta 4 (build 21A5291j), Go Button now receives an error -54 from iOS when loading an audio track from the device Music library. (Error -54 translates to a file permission error.) The same code running on iOS 16 and earlier does not cause an error. And as you've found, loading files that were imported into the Documents folder also does not generate this error.
We know of at least one other major iOS music app that is experiencing the same error, so this definitely is not unique to our implementation.
We have filed a report with Apple and escalated it to Developer Technical Support. At this point, we don't know if this is a bug or a new policy. We may not know for another month or so if it will get fixed before the iOS 17 public release or what might be a potential workaround, if any.
(This is a good time to repeat our blanket recommendation that, generally speaking, it is much better in the long run to import your audio files into Go Button than use tracks in your Music library.)
We'll be sure to post an update if we get new information.
Brent