This drag-n-drop script for windows merges mp3 Files in multiple folders by dragging them onto this bat file. The result is one merged mp3 file per folder.

merge_MP3_folder.bat

@echo off
chcp 1252

::===============================================================
:: This drag-n-drop script for windows merges all mp3 Files in a
:: folder by dragging it onto this bat file. It can be used for
:: multiple folders at once which outputs one merged mp3 file per
:: folder.
::
:: Installed ffmpeg is required. To use ffmpeg you need to install
:: it on your OS. Look here for instructions how to do this:
:: https://video.stackexchange.com/a/20496
::
:: Alternatively you can replace the ffmpeg command in this
:: script by the location of your ffmpeg executable.
:: e.g. C:\Programs\ffmpeg\bin\ffmpeg.exe
::===============================================================

SETLOCAL EnableExtensions EnableDelayedExpansion
pushd "%~dp1"

echo ### Merge mp3 files in folder ###
echo.
echo Current working directory (%%CD%%): %CD%
echo Directory of this script (%%~dp0): %~dp0
echo.

echo Folders will be processed in this order:
for %%z in (%*) do (
	echo %%z
	)
echo.

set back=%cd%

for /d %%i in (%*) do (
	echo %%i >>folders.txt
	)

for /f "tokens=*" %%i in (folders.txt) do (
	rem	echo %%i

	echo Processing Folder: %%i
	cd %%i
	echo Files will be processed in this order:

	for %%f in (*.mp3) do (
				echo %%f
		set "datei=%%~nxf"
		set datei=!datei: =\ !
		echo file !datei! >>confiles.txt

		if not exist "metadata.txt" (
			ffmpeg -i "%%~nxf" -v quiet -f ffmetadata "metadata.txt"
			)
		)

	set "datei=%%~nxi"
	echo using ffmpeg...
	ffmpeg -f concat -safe "0" -i confiles.txt -i "metadata.txt" -v quiet -map_metadata 1 -id3v2_version 3 -write_id3v1 1 -c copy "!datei!_merged".mp3

	echo !datei!_merged.mp3
	move "!datei!_merged.mp3" %back%
	del confiles.txt
	del metadata.txt

	echo.
	)

cd %back%
del folders.txt

echo Exiting ...
echo.
pause