I can't try it on Win atm and would need to install mpeg, etc. This is working or me on Linux.

  for f in *.mp3; do 
    ffmpeg -i $f -filter:a 'volume=1.5' test/$f 
  done

That would loop trough each mp3 in the current directory and save the files using the same name in the (already created) test subdirectory.