This is because of the Fraunhofer MP3 codec (the highest quality MP3 encoder/decoder) has hard-coded into it a 'mute time' at the beginning and end of the file. This cannot be disabled or changed within the Fraunhofer codec, so there is no workaround within Sound Forge (and any other programs which use this codec). You can, however, use another MP3 codec, such as Xing or Lame, available on the internet, which do not add this silence to the file. These are often bundled with free software available for creating mp3s on the internet.