Level 5: Type in 777. In the window type 885497. Just look at the source, download the second mp3 file and listen to it backwards. :<
Level 6: Now, ignore the error page and look at the source again. Take the hint and add "?whatdoesmath=cow" at the end of the .php thingy in the address bar. Now You get an ! in the window. Look at the source again. You'll see 'firehouse.php' at the end. Now change the address bar from level6.php to firehouse.php. Enter formula here. And then cow. Now enter 885497 again. Then remember level 1 and enter ram.