Before applying this file, make a backup of "TheMonstersWithin.wav" original soundfile, then copy and paste the one downloaded.
"...\Valve\Left 4 Dead\left4dead\sound\music\unalive"
Once you copied and pasted, run L4D, open your console and type "sndrebuildaudiocache" and "sndrebuildlocalizedaudiocache". Don't remember if the last one was like that, but its similar. When it's finished try it and comment any issue!
Any new ideas are welcome.