Batch file if statement error

[gameman]

It may be something silly but I keep getting the same error
"File Doesn't Exist!"

:loop
set /p song!a!=Enter song name %a%:
if not exist %song!a!%.wav (if exist %song!a!%.mp3 ( set song%a%=%song!a!%.mp3 ) else (Echo File Doesnt Exist!
set /a a=%a%-1))

set /a a=%a%+1
if %a% GTR %sngs% goto endloop
goto loop
:endloop

Edit
I found out why! for some reason the if statement doesn't like %song!a!% but it does like !song%a%!. I fixed it by tinkering with it.