Rather than start yet another telltale thread:
I've just released two more Telltale-related programs - see here for more info. (
http://www.lucasforums.com/showthread.php?p=2138639#post2138639)
While I was making Telltale Explorer, I suddenly remembered Telltale's first game - Texas Hold'em. Sure enough it also uses a .ttarch archive, in Texas Hold'em though - nothing is encrypted. The prefs.prop file in the root folder doesnt have an encrypted header (as it does in other versions) and none of the files in the .ttarch are obscured. Its even got the original lua scripts in there in plain text format.
In the absence of a decrypter for the annoying 'pattern' and header encryption, the files in Texas Hold'em are probably the best starting point for anyone looking at the various file formats.
[Edit] Texas Hold'em also confirms what I suspected - that the voice files are encoded with the speex codec. Unfortunately it seems that the only way to play these back is to embed them into an ogg container - does anyone know of another way of playing speex stuff back?