There was an older issue with Flash CS6 where any MP3 with a bitrate over 192kbit could not be imported unless both QuickTime and iTunes were installed to the latest version (on Windows). Do you also have iTunes installed and updated?
I don't think anyone got to the bottom of why iTunes was needed but many people couldn't import MP3 with the same error until they did. You might want to try that.
The only MP3 workaround was to import directly to library instead of stage and play it in a timeline layer, but some could only hear it during published playback (no preview).
Of course an alternate workaround is to simply use a different format. There's still something going on with MP3, which is a bit surprising considering how common it is. I would recommend trying WAV or AIFF as a workaround for now if iTunes/QuickTime updating doesn't fix it.