If you have a surround setup, make sure that both the console AND the game are set to the same output. For example, if your Xbox is set to 7.1, make sure the game is set to 7.1 as well.
If you don't have a surround setup, then YOU SHOULD NOT SELECT A SURROUND OPTION. For people saying that the dialogue is very faint, it's possible that you're just hearing the reverb from the dialogue. This is because dialogue is sent to the center channel, and if you don't have a center channel, then you won't hear it. Try setting your console output (in Xbox settings) and the game output (in options menu) to stereo. Not a guaranteed fix, but with the info I have, that's my only suggestion.
@hypo92 if switching from "5.1" to "2.1" solved the problem, that would lead me to think that your surround system is not set up properly. In other games, it could be the games themselves that are outputting incorrectly. Many games don't truly support surround, and it can be hard to tell if you're listening to the intended mix or if audio is outputting to your rears unintentionally because the developers couldn't test it.
There is a calibration test on start up of the game where you can select your audio output. Listen to the tones that are output. Each tone should be in an individual speaker! If it's not then something is messed up with your system.
To add, "2.1" is actually not supported. It's just stereo. For people with "2.1" setups, this is essentially just feeding the audio content to the sub, but only audio content below the sub's crossover frequency is actually outputted through it. It's not a dedicated LFE.
Finally, for dialogue/sfx falling out of sync, is there any repro for that? That seems to be a code thing, but if there is no repro then they will not be able to fix the issue.
