@echo off
set ext=mp3
setlocal ENABLEDELAYEDEXPANSION
set path2=C:\test
set a=1
dir /s %path2%.\*.%ext% /b >%path2%.\%ext%.dir
for /f "delims=" %%a in (%path2%.\%ext%.dir) do (
set b=!a!
if !a! lss 10 set b=0!b!
ren "%%a" !b!.%ext%
set /a a=!a!+1
)
del %path2%.\%ext%.dir
C:\test is the folder of the mp3 files