Так заработало:
@echo OFF
SETLOCAL ENABLEDELAYEDEXPANSION
for /L %%i in (1,1,5) do (
for /F "tokens=2" %%a in ('ffmpeg -i vidov_ska4ali_%%i.mp4 2^>^&1 ^| find "Duration"') do SET z=%%a
set /A Z1=1!Z:~0,2!-100
set /A Z2=1!Z:~3,2!-100
set /A Z3=1!Z:~6,2!-100
set /A vidov_ska4ali_%%i=!Z1! * 3600 + !Z2! * 60 + !Z3!
)
echo file "vidov_ska4ali_1.mp4" have 12 sec but displays !vidov_ska4ali_1! seconds
echo file "vidov_ska4ali_2.mp4" have 21 sec but displays !vidov_ska4ali_2! seconds
echo file "vidov_ska4ali_3.mp4" have 9 sec but displays !vidov_ska4ali_3! seconds
echo file "vidov_ska4ali_4.mp4" have 13 sec but displays !vidov_ska4ali_4! seconds
echo file "vidov_ska4ali_5.mp4" have 14 sec but displays !vidov_ska4ali_5! seconds