I have the same problem. Oddly enough, the sound almost always stops at different times. The narrator cuts out at random, and for the in-mission briefings, that usually short-circuits the camera motion as well. But not always: One time the camera began zig-zagging about, giving me a very silent view of the battlefield.
I've made it through the full first-mission briefing once, but that's it so far. All my other attempts have halted midway. One thing I have noticed: selecting "restart mission" almost never resets the briefing, if it was cut off. It usually requires that the game be restarted entirely.
A look at the game's briefing files shows that they're all stored in segments as tiny .wav files. 6 or more for each mission, it looks like. I'd guess something is wrong with the game's ability to schedule the start of one .wav after another has finished, but I'm not a computer whiz, so I have no idea why it would do that.