I have 21 files to import to audacity that are currently joint stereo mp3’s that I need to convert to mono mp3. I know how to import the files but do not know how to convert them. I also know how to export but think I need that conversion step in between. Help please as my author is waiting for the audiobook to get publish on ACX.
Thanks.
If you still have backup copies in WAV format, or as Audacity projects, then I would highly recommend using those rather than the MP3s for making the new mono files.
MP3 is a “lossy” format - that is, some of the audio data is discarded so as to make the file smaller, thus encoding as MP3 always reduces the sound quality.
If you only have the MP3s, then you can use those (but make a note to self that it is worth keeping high quality WAV format backups of everything 
)
To convert a file to mono you can use “Tracks menu > Stereo Track to Mono”
The quickest way will be to import several of the files together (using the “Import” command in the File menu). Then select all of the imported tracks (“Ctrl+A”), then “Tracks > Stereo Track to Mono”.
If you need control over the metadata for each file, Export the tracks one at a time by selecting the track, then “File > Export Selection”.
If you are not using metadata, then the quicker way is to use “Export Multiple” http://manual.audacityteam.org/o/man/export_multiple.html