Did you make any progress with this? I'm having the same trouble. I'm unable to open the file URL with ExtAudioFileOpenURL(), and startAccessingSecurityScopedResource() returns false.

That makes sense, but how to I gain permissions to access the file? It's an attachment to the INSendMessageIntent that my app is handling, so there should be some way for my app to gain access to it, right?

It looks like security scoped bookmarks are a macOS-only feature. I'm running into this issue on iOS. Is there an API that I'm not seeing? I created a ticket in Feedback Assistant (FB16091226)