If you are planning on commercializing whisper, this seems like a perfect opportunity to put yourself in a better position than your competitors. Rather than place a warning. I truly believe you can prevent this issue from occurring with just a little bit of elbow grease.
Usually, these kind of features can be expanded as well. If you are monitoring the strength of the audio, you can display it like Whatsapp and other messaging apps do when you create a voice note.
It would be very easy to anticipate a hallucinated ending based on the audio sample you have shown