What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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May I know the file size and the file name/extension of the mp3s file you're trying to upload. I can see the that upload field is configured properly. It is possible that the upload file size is too small, you might need to increase it.
I cloned your form and did a test submission with an uploaded file of 3.84mb .mp3 file. I was able to submit it without any problems.
We'll wait for your reply. Thank you.