I don't think you'll ever find this in true FLAC since there's no OST & game itself uses pretty lossy audio on both platforms but there's still a solution: DL this (
http://x360.joshw.info/l/Left%204%20Dead%20(2008-11-17)(Valve)[X360].7z), drag & drop *.xma files to towav.exe (just google it & you should be fine) & here you go - you get PCM WAV conversions! Of course source sound files weren't even close to lossless but you'll be able to encode those conversions to whatever codec you want (and your audio player supports) without a drastic loss in quality.
Best regards!