When holding up/down with the pasted code you can see a subtle "cracking" when the played sound is loud. This has been tried on different machines and different videos to make sure it is not hardware related. This has been tested on Linux. The video.ogg file is big_buck_bunny_480p_stereo.ogg but as said this happens with different files.

import QtQuick 1.0
import QtMultimediaKit 1.1

Video
{
focus: true
id: video
source: "video.ogg"

Component.onCompleted: video.play()

Keys.onPressed: {
if (event.key == Qt.Key_Up)

{ video.volume = Math.min(video.volume + 0.05, 1) }

else if (event.key == Qt.Key_Down)

{ video.volume = Math.max(video.volume - 0.05, 0) }

}
}