I'd be happy to take a closer look to see how we can help. As a first step, during the playback of any content, press the * key on your remote. A menu will open on the lefthand side of the screen. Do you have any 'Volume mode' enabled here? If so, try turning this off. Does this resolve the issue?
Are you using an audio/video receiver of any sort in your home setup? Is your device connected directly to the receiver, or to your TV?