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So far audio from HL2 we have some audio that when played backwards is significant.

The G-Man's TV song played backwards (radio1.mp3) is the SAME as forwards, except his laugh can be clearly heard.

There were two threads about the zombie cry, too (Thanks to both posters of the thread. Not taking credit for any of this, or for the reporting of it.. in which they said they didn't take credit either. lmao) -
Originally Posted by reefsurfer
Original ingame sound/moan.
http://www.bigoindustries.com/HL2/Audio/moan_loop1.wav

Played backwards.
http://www.bigoindustries.com/HL2/Audio/godhelpme.mp3

G-Man
Zombie

Find anything else and we can add to the list. Perhaps this is all but if not it'll be useful.