Messed Up Vinyl

You can do that with SoX

SoX is a command line program and does not include a batch processing function, but you can use almost any scripting language to run SoX as a batch process. For example on Windows you could write a .BAT file, on Linux you could write a Bash script, on any platform you could write a Python script.

If all of the files are exactly the same format, then that simplifies the task considerably.
To convert from a stereo track to a mono track using the left (first) channel only, the command is:

sox "stereo.wav" -c 1 "mono.wav" mixer -1

where “stereo.wav” is the name of the input file and “mono.wav” is the name of the output file.

There’s some examples of writing batch scripts here: Batch Scripts with other programs
You may want to look on Google to find some examples for your operating system.