There are two issues found in QML MediaPlayer type on iOS 9 devices:
1. It cannot play remote mp4 and mp3 file at all - no video, no sound
2. When playing local mp4 file (from qrc resource), it only showed video, but no sound.
(These two issues do not exist on Android)

Following are the test code used:
For issue 1:
import QtQuick 2.6
import QtQuick.Controls 2.0
import QtMultimedia 5.6
Item {
MediaPlayer

{ id: mediaplayer source: "http://clips.vorwaerts-gmbh.de/big_buck_bunny.mp4" }

VideoOutput

{ anchors.fill: parent source: mediaplayer }

MouseArea

{ id: playArea anchors.fill: parent onPressed: mediaplayer.play(); }

}

For issue 2:
import QtQuick 2.6
import QtQuick.Controls 2.0
import QtMultimedia 5.6
Item {
MediaPlayer

{ id: mediaplayer source: "qrc:/big_buck_bunny.mp4" }

VideoOutput

{ anchors.fill: parent source: mediaplayer }

MouseArea

{ id: playArea anchors.fill: parent onPressed: mediaplayer.play(); }

}