Yes, subliminals work in MP3 format if produced correctly. In fact, it is the format we use for our subliminals. We have done extensive testing and found that MP3s are effective and the most user-friendly for most people.
Subliminals can be produced in many other formats. Some become huge files and would take up a lot of space on your phone or devise. Not to mention huge files take a long time to download or upload to your phone.