Convert Several .au files into 1 MP3 file

I have an issue where several recordings were saved as projects. When I attempt to play the .au file I get nothing. Is there a way to recover these files
( I cant open from recent files due to the amount of time that has past since they were originally recorded)
I realize these files should have been exported to MP3 instead of selecting the save project as.

Is there a way to combine all of these files and export them out as an MP3 or WAV file? or am I out of luck?

Audacity saves all the little au files (which aren’t all sound) when you Save a Project. The script or list of instructions to put the show back together is the AUP file. Double click on the AUP file and it will launch Audacity and your show will appear. Without the AUP file, the chances of success go way down. If you Edited the work before you Saved the Project, then the chances of recovery without the AUP file is zero.

I realize these files should have been exported to MP3

Actually, they should have been exported as WAV files. WAV is the perfect backup. From there you can make MP3 or anything else. Once you make an MP3, that’s the end.