These are the audio formats that are compatible with iMovie. As you can see, Mp3 is among those formats that iMovie can use.
Audio formats compatible with iMovie:
- AAC
- AIFF
- BWF
- CAF
- MP3
- MP4
- RF64
- WAV
However, sometimes Mp3 can be a bit more problematic than other formats. So, you might try converting the clips to one of the other formats, like AAC or aiff.
Also, where are the Mp3 files residing, and how are you trying to import them?
The first thing that I would try is deleting/resetting preferences. To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes. Then see if you can import your Mp3 files. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
If still no luck, try creating a new project and importing into the new project.
-- Rich