I am running Windows 7 and Audacity 2.0.6 (I had same problem on 2.0.4)
I create a file (stereo) by recording a sound, importing a sound or copying a sound from another Audacity file. I then close the file and then re-open it later for further use only to find that segments (usually small) spread throughout the file have no sound wave on one channel of the stereo. If it is no sound on the bottom channel of the stereo then the sound still plays but if the sound is zeroed on the top channel of the stereo sound then the sound does not play.
Was the imported sound a WAV file and you subsequently moved, renamed or deleted the WAV? If you want to do that, open the Import / Export Preferences and choose the option to “Read uncompressed audio files directly”.
Similarly, don’t move or rename any of the little AU files in the _data folder of saved Audacity projects (and also leave alone the AU files in the Audacity temporary folder before you save a project).
Open Help > Show Log… to see the names of the AU files that are missing and the exact subfolder of the _data folder that those files are meant to be in. You can right-click in the log > Select All then right-click again and copy. You can then paste that information somewhere to refer to.
Then close the project, find the files by searching your computer, put them where the log says they should be, and you should be able to open the project properly.