It’s tough to know exactly what’s going wrong in your app, but if what @Beatrix_Willius is saying is true, you likely need to put in quite a bit of work to modernize it. On 2019r1 you won’t be able to build for Apple Silicon either.
I can definitely confirm that AVFoundation and r/w access to Application Support still works on Sonoma, so there must be something else going wrong.
You will need to fire up Xojo on a Mac running Sonoma and work through it step by step (alternatively you can use the Remote Debugger). I would highly recommend updating to a newer Xojo version as well so you can compile a Universal Binary (2023 R4 also added some pretty serious speed improvements for Windows too). The platform licenses are on discount for $99 right now so maybe the stars are aligning.
Edit: Apparently there are some issues running Xojo 2019 R1.1 on Big Sur or later as well, as is being discussed in this thread. So it may just be time to bite the bullet and upgrade.