Your file is fine, it’s just [Google STT V2 doesn’t support M4A files](https://cloud.google.com/speech-to-text/docs/encoding), even though it looks like it. It is like charging your phone with a non OEM charger that should work but it does not. Even tools like `ffprobe` say the file is ok, but Google silently skips it.

What you can do is convert your file to a different but supported file like .wav or .flac and submit it to Google STT and it should work.

This is interesting to be available natively. On Google side, there is a feature request that you can file but there is no timeline on when it can be done.