I had this problem first with the beta and now also with the retail. I solved it by copying over the eax.dll from STALKER(in the bin directory) over the one in Assassins Creed.. If you dont have stalker installed, google for eax.dll version 3.0.1.0 and Im sure you will find it.. It solved the voices/sound issues for me but I cant promise it will work for everyone.. 
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[INTEL C2D E8400@4GHz] [ASUS Rampage Formula] [PowerColor Radeon HD 5870 PCS++]