I have played Mass Effect 2, S.T.A.L.K.E.R Call of Pripyat, Borderlands, and Just Cause 2 (only games i can think of that have this problem)
and the dialogue that the characters speak get cut off when there is a lot of dialogue said (around 2-3 sentence i think).
I have a Realtek ALC888 chip in my mobo which is the GIGABYTE GA-EP45-UD3L.
I would really like it if this problem is solved, because it does get really annoying when you have multiple games that have this problem.
Make sure you've your speakers properly configured with windows.. That means if you have a two speaker setup then make sure its configured in windows sound scheme setup as stereo and not as some 5.1 setup etc..
Intel Core 2 Duo E8400
XFX Ati HD 5850
I also use head phones for my sound but when i hooked u speakers it still does the same. Also i did try to change it from 7.1/5.1 to Stereo/Quadraphonic but no change ( I changed this by using the Realtek HD Audio Manager)