The first thing I checked was whether or not the computer recognized MP3, which it does. What is just weird is how this install of Sound Forge is not recognizing mp3. It has the MP3dll in the right folder, etc. It offers MP3 as an associated file type. But when opening or saving it does not offer mp3. When I select all files and open an MP3 file, it is reproduced at 1000% speed or so it seems. So something's amiss. I've opened a support ticket. I can't imagine this has not happened before. Thanks much.
Have your tried re-setting SF to the factory default settings (though II am not sure Pro12 or AS has this.
You could also try uninstalling Sound Forge with a third-party uninstaller that can remove leftover register entries, folders and other pertinent data files the OS uninstall leaves behind. Then reinstall SF. I like Revo Uninstall. There is a freeware version as well as a pro version. The freeware version is adequate for most folks.