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I took the liberty of uploading the original ogg files to mediafire resampled to a 44.1kHz samplerate. I'm pretty sure I removed all duplicate tracks and you can convert them to whatever format you please.
Put the DoomMetalVol4.wad in your skins folder in Zandronum.
Awesome mod, it just seems to me they are for some reason sounding just a little too soft and it must be the game mixing that's to blame. I'm using this on L4D2 I haven't tested it on L4D yet but watching the video it seems it should be a whole lot louder.
I already maximized the volume of the files with some (good) sound processing and it didn't do much in-game.
If you want I could straighten out all the files to have more or less the same sound/volume, there's a few 'bad' ones in there.