Not sure about Iphones, but my android uses the same volume button for media sound as it does for call volume. You control it depending on what mode you are in. It might be ok when you are on the phone, but when you go to media, it is off. Also, when playing something back, even though you have the volume on on your device, it may be on mute on the video. You can check that by bringing up the play/pause bar at the bottom of the video. It will show a speaker. If there is a x next to it, the sound is off. It also could be the sound is all the way down on the video also. It will show as a vertical bar right above the speaker.