Same here, the sound cuts in and out at exactly the same spots every time. Updated drivers etc etc but still the same.I also have the crashing problem on the second level helicopter bit like alot of other people with an nvidia card. Wish I could take it back as I cant even play multiplayer, it crashes that often I get banned for 'rage quiting'. Not impressed.

I'm having the same problem during sinle player in the same place which you mention and also in multiplayer where it warns player about things such as lightning strikes and UAVs, i'm not sure about zombies as i havent checked yet but i'm sure that the problem will occurr their aswell. When i first got the game on release day i could hear everything as normal but in the days to follow the voices have gone as you have mentioned in the original post. To try snd resolve the problem it spoke to the manager of the game store where the game was purchased and he had heard nothing about this and told me to wait and see if a patch was to be released to fix this problem, but he said it was unlikly until thousands of people complain about this problem.

This is what Activision have suggested FOR XBOX 360, it's not a lot to go on and doesn't really help but it's a start.

https://support.activision.com/articles/en_US/FAQ/Audio-Is-Distorted-or-Missing/ ?l=en_US&c=Game_Title%3ACall_of_Duty_Black_Ops_II&fs=Search&pn=1

If the information provided in this link works for you then please leave a comment on which route you took and any that didn't work, any information would be greatly appreciated. Thanks.