I hear the cracklings as well. I don't think they have anything to do with the headphone, since I have tried 4 different sets and have the same issue with all of them.
Do you know what is the bitrate and sampling frequency of the MP3 files you are trying to play ? It may be an issue with the Kindle not able to handle very high bitrates (the MP3s I have tried are all encoded by me at 320kbps).
This could also explain why it happens for some MP3s but not all.
I will re-encode some of the tracks with a lower bitrate (anywhere between 128kbps and 256kbps should be fine in most cases) and try again.
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