Many users have much of their music "library" in the MP3 format. These are heavily-compressed Audio versions of the MPEG CODEC. Besides just the music, or SFX (Sound Effects), they can contain other data as well, like "album art."
When working with these MP3 files, the NLE (Non Linear Editor) program must process them, and it takes a bit of work. Some of that other data can cause problems, or even hangs, either on Import, editing or perhaps output.
The best workflow, IMO, is to convert all MP3's to PCM/WAV (not "Linear PCM," or LPCM, but pure PCM - Pulse Code Modulation), at 48KHz 16-bit. Those WAV files will Import, edit and Export perfectly, and nothing is better to use in an NLE program. So many problems, and potential problems can be circumvented with this workflow.
There are several easy ways to do this conversion. There are typical conversion programs, that can handle Audio, as well as Video. Or, one can rely on an Audio-editing program, like Adobe's great Audition, or even a free Audio-editor, like Audacity. Both are stand-alone products, though Audition can be directly accessed from within PrPro, with a little bit of File Association. Note: as of Audition 3 (current now), that program is PC-only, but Adobe is going to release an X-platform version, probably Audition 4, so it will be available for the Mac, as well as the PC. There are also many other Audio-editing programs, that can convert MP3's to PCM/WAV 48KHz 16-bit files. The choice is up to the user.
Hope that this helps, when dealing with MP3 Audio material.
Good luck. I recommend using the audio-editor, with which one is most comfortable. If one does not have an audio-editor, then Audacity is a good one, and it's free. There are several tutorial articles on using Audacity at Muvipix.com. I use Adobe Audition, because I know it, like it, and it works beautifully for me. However, it is not free, though is included with several Adobe suites, and has recently been ported for the Mac.