The first piece of information we need to find out is whether there is any sound from the 360 at all. When you switch menus in the dashboard, do you hear anything?
Do other games have proper sound?
Which game are you trying to play? Some games give the option to alter what sound setup you are using. If your current sound setup does not support 7.1, 5.1 surround sound, etc.. you will not hear anything.
Go to your dashboard.
Press the Right Bumper until you get to the "System" tab. Select "Console Settings".
Select "Digital Output".
Select "Dolby Digital".
Any sound now?
And, as with any audio problem, we need to make sure your audio cables are plugged into the proper jacks. What type of a/v output cables are you using on your 360 (i.e. what color are the tips?).
Thanks, reply back with any information you have, please!