Not all mp3 files are created the same, and information in the file header CAN cause problems
Convert to WAV and try again
What version of Premiere Elements are you running on your Windows 7 64 bit?
Be that as it may, I suspect that you have iTunes music. Your iTunes music issue is a classical one.
If my assumption about your music source is correct, then please take your .mp3 music files
into iTunes and convert them into .wav files. To do that
1. Import your .mp3 into iTunes via File Menu/Add File To Library.
2. Then Edit Menu/Preferences/General and click on the "Import Settings" tab to bring up the
Import Settings dialog. In the Import Settings dialog, set Import using: field to WAV Encoder.
3. In iTune Music, right click your imported .mp3, select "Create WAV Version" and OK out of
4. Right click the wav version that has been created, select Copy, and then paste the copy
to the computer desktop.
5. From the computer desktop, import the .mp3 into Premiere Elements with its Add Media (Get Media, if version earlier than 11)/Files and Folders.
Please let us know if that worked for you. If it does not for some reason, please give more details.