You can still change the music / sounds of infected/bgm/weapons/etc. Just search it on Google and replace the music files on your steam folder. Once you replaced your sound files, start L4D2 and type in the console snd_updateaudiocache, once updating is done, open console again and type snd_rebuildaudiocache. The game will freeze for a few minutes, but it's normal. Once done, reboot game and enjoy the new sounds of your game.