Use a Section field (or the form's description or the success messages), go to the View option in its menu, and open the Source code option. There you can add the following code and save it.

You should paste the link to your custom audio in the code below. (You'll need to upload the audio somewhere and paste its public link in the code) Here's an example:

The code to copy:

<p><audio src="PUBLIC LINK TO YOUR AUDIO" controls="controls"></audio></p>

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