Ok, resolvi o problema usando o seguinte arquivo .BAT.
@Echo Off
SetLocal EnableDelayedExpansion
CD /D %~dp0
Set Count=0
For /F "tokens=*" %%f In ('Dir /A-D /B *.flv *.mp4') Do (Set /A Count=!Count+1)
Set /A Skip=%Random%%%%Count%
If %Skip%==0 (Set Skip=) Else (Set Skip=skip=%Skip%)
Set RandomFile=
For /F "%Skip% tokens=*" %%f In ('Dir /A-D /B *.flv *.mp4') Do (If "!RandomFile!"=="" SetRandomFile=%%f)
Start "" "%RandomFile%"
EndLocal
Obrigado pelo interesse:)