vlcmediawidget: unmute audio when the player instance is created

As reported on bug #376195, when Kaffeine is called after
a past instance having it muted, the player starts on mute
state.

Fix it by forcing it to unmute audio when the VLC instance
is created.

BUG: 37195
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>