The voice audio mixing isn't very good. I noticed this issue today also.
So apparently the Xbox switches to Surround 7.1 on its own. I switched it back to Surround and everything works now.
I am having the same exact issue daily. Xbox knows nothing about it. Please call the and report the issues so we can make sure it's not our hardware.
Can confirm that changing your surround sound settings will fix the problem, atleast it worked for me :)