I have noticed that, with Audacity 2.0.5, I cannot set any file defaults other than Audacity project files. I have tried to add a default that all Ogg Vorbis files would open in Audacity, but Audacity does not show in the list of recommended programs. I browse for Audacity manually, and when I try to set it as the default for the specific file, Windows just goes back to the recommended programs window. I am using Windows Vista Home Premium with SP2, and I have access to administrative permissions.
I presume that you previously had an older version of Audacity installed and set up file associations for that?
There is a “limitation” (bug) in Windows, that the registry cannot handle file associations for a program if that program, or a program with the same name, was previously installed in a different location.
An easy solution is to completely uninstall Audacity, then use a safe registry cleaner (for example, the registry cleaning tool in CCleaner) to remove the (now invalid) file associations. The default options in the CCleaner registry tool will do this. Then install Audacity 2.0.5.
Note: The registry is critical for Windows. Before cleaning or otherwise modifying it, make a backup of the registry. CCleaner will offer to make a backup before it changes the registry.