exception in vorbis

System.IO.FileNotFoundException was unhandled by user code
  HResult=-2147024894
  Message=Could not load file or assembly 'System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.
  Source=NVorbis
  FileName=System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
  StackTrace:
       at NVorbis.VorbisReader..ctor(Stream stream, Boolean closeStreamOnDispose)
       at BasicEnglishWords.Views.SomaPage.<button_Tap>d__3.MoveNext()
  InnerException: