The temporary files used with MP3 Audio Mixer may have become corrupt. Run MP3 Audio Mixer. Select "Preferences" from the "File" menu Then select the "Miscellaneous" tab and click "Delete Temporary Files". It will ask you "Are you Sure?", click "yes". Shut down MP3 Audio Mixer, start it again and load in your saved mix or SGP file.
If the problem persists try choosing a different temporary files directory in the Miscellaneous preferences.