Try clearing the cache on the xbox 360.
To clear the cache on the Xbox 360 do the following:
Go to the Xbox Dashboard, select the System Blade, and select Memory.
Highlight the HD symbol and press Y.
Press X, X, Left Bumper, Right Bumper, X, X.
You will see a confirmation message to confirm system maintenance, proceed.
Note: Clearing the cache on the Xbox 360 is a fairly simple process, and can be useful if the system seems sluggish, unresponsive, or you just need to get rid of a game update (for whatever reason).
The most important part of clearing the cache is knowing what to expect. All downloaded game updates will be removed. So if you have downloaded an auto-update for a game, in order to play it on Xbox Live you will need to re-download it.
I have done this twice (Oblivion, Fallout 3) and it made a huge difference. It is worth a try to see if it will get rid of the sound issue you are experiencing.