Looks like a recent update to the backwards compatibility list might have cause more problems than it fixed, as it added audio errors into the Xbox games played on the 360. Thankfully, Microsoft has already released another update to fix the problems that the original update created.
“Today we’ve released a compatibility update to resolve the loss of audio problem some customers outside of North America and Asia were experiencing with the March update. The Backwards Compatibility product is complex and we committed an error when building the package to release on Live. These issues are now resolved.”
The update was released late last night but should be available for download now.