I was looking for something similar for mp3's to aac+ a month ago , 15gb / 5000 mp3's of music with likely 20% or more duplicates with similar or different names and some junk quality ones
Just ended up deleting most of it.
Possible future add on's :
Do you need to add something to check if needed codes is installed ? and or install them if not.
Might want to add something to create a folder and then check for enough space on hd or folder to dump the files into , maybe something to clean the temp files afterwards .
Presuming it takes same space as original format to calc space requirements give warning and then continues until disk full or task completed without stopping the process.
Progress bar per file and total as this will take ages depending on volume of files .
Resume / pause task where it does not do same files over , MD5 check for completed files or something , file format limiting ie: exclude known file type if video ones is too many to list / formats , exclude mp3 wav ?
Then can later add specific commands for each file type provided the command line converter has this options.
Then make a gui interface and sell it LOL.
Heck can likely use spritz and modify it to be a gui for this ?