try this
echo off
setlocal
set "INPUT=input.mp4"
set "MUSIC=music.mp3"
set "OUTPUT=output.mp4"
for /f %%a in ('
ffprobe -v error -show_entries format^=duration ^
-of default^=nokey^=1:noprint_wrappers^=1 "%INPUT%"
') do set DURATION=%%a
for /f %%a in ('
powershell -NoProfile -Command...