Hi there @sorenzi! 👋
Is the URL in the Files Permalink field publicly-accessible or do you need to be logged in to be able to access it?
If a login is required to access the file, the Zap might be getting redirected to a login page when it tries to access the file. Since that login page isn’t a valid file URL, it could explain why the Zap is erroring, even though a .m4a file was selected. You can learn more about sending files here: Send files in Zaps
I had a look in my own Zap and was able to find a field that contains the file object which should allow the Zap to send over the actual file:

In the File field on that Create Transcription action, can you try selecting that same Files 0 File field in your Zap and let us know if that fixes it?
Hi @sorenzi, just checking in! 🙂 Did my suggestion help to fix the Zap?
Want to make sure you’re all set here, happy to help further if you’re stuck at all.