@y.ashibe Apologies for the late response. I appreciate your patience on this.
I have got a reply from the Product owners. They have analyzed the backend logs based on the details which you had shared.
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Cause:
The problem with the file is that the metadata is at end of the file. The concrete decoding error we get is that no moov atom can be found within the first 10MB. If the audio encoding subsystem cannot determine the format we cannot process it. This should explain why 1-2m files work since the file is below 10MB, hence the metadata being found.
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Action Plan:
The recommended way to fix the issue is the preprocess the audio, e.g., with ffmpeg
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If you have any follow-up questions, please feel free to ask. I would be happy to answer it.