Go to Edit → Preferences → Modules. Depending on your version of Audacity you may see several mod-xxx boxes for various audio formats. Enable them all.
Or you can delete your audacity.cfg file and when you re-start Audacity it should build a new-clean one. This page tells you where to find it.
When you un-install it keeps the audacity.cfg file. Then when you re-install it uses the old settings by default. (I think it asks a question about that.)