Try deleting the file called "Acoustica.adb," which is located here:
%appdata%\Acoustica\Acoustica MP3 CD Burner\
You can copy and paste that pathname into the Windows Explorer address box to pop it up.
After you delete the Acoustica.adb file, run the MP3 CD Burner program and see if the problem is fixed. Acoustica.adb contains Music Library reference information and when you delete it and run the program again a brand new Acoustica.adb file is created. Click the Scan button in the Music Library to fill up the library again (if necessary).