I export .aup3 files to MP3 and then can’t open them. I have followed Microsoft guidance with a command prompt to sweep the system to look for missing files. I have also uninstalled and reinstalled Audacity (I’m already working on the most recent version, 3.7.5. I have searched and can’t find this topic or support anywhere. Help!
You’re only the second person in the history of this forum to report that error message …
https://forum.audacityteam.org/search?expanded=true&q=%20%22Class%20not%20registered%22%20
Could check mp3 module is enabled in Audacity, (if the problem is specific to mp3 in Audacity) …
Thank you for responding. Yes, the mp3 module is enabled in Audacity. I also just installed version 3.7.7 and it is still not working. I still can’t open the file. Any other suggestions? Thanks again!
I found the solution - I needed to change the default media player in Microsoft back to the “legacy” version. I guess the newer Windows Media Player has issues. 
This is a peeve I have about the current Microsoft media player. If you don’t have a codec to play something, it will ask you if you want to install that codec. If you are pressed for time, or mistakenly tell it no, it will never ask you again in the future, even if you later tell it you want to play a file needing that codec.
It is another reason people install a codec-less media player like VLC.
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