Welcome to the forum.
MP3 Audio very often causes problems. Sometimes little ones, but sometimes big ones.
My advice is to always use an audio-editor, like Adobe Audition, or the great, free Audacity, to convert the MP3 to PCM/WAV 48KHz 16-bit, and Import and use that in PrE. With the exception of tiny SFX MP3's, I always do the conversion. All you have to do is Open the file in the audio-editor, and then do a Save_As, choosing the specs. above.
Also note that the program must conform and create peak files for certain media files when you first add them to your project. (You can see the progress in the lower right corner of the interface.) If you interrupt this process and try to work with these files before they're completely conformed, they may only be partially conformed and you get only half the audio.
Try clearing the audio from your Project panel (from the right-click option) and then re-adding it to your project -- making sure to allow the program to completely conform the file before you start working with it.
Steve! that did it! Thank you for your help.