My XBOX loses all audio when playing a video from a USB device. Both the video and the XBOX system (when pushing the button on the controller) don't play any sound. However, if you exit the video, the sound is back.
I have tried with different USBs and files so we can rule out a problem with them. Games' sound, console sound, settings, and the HDMI cable are fine. If the XBOX video player have some kind of audio settings separated from the console audio settings I cannot find them.