Looks like there is a properity for the media player called "Settings" and a property for settings is volume.
Couple comments though, you have the media player disabled, if you leave it enabled, you can use the default controls on it.
Secondly, they way you have this program set up is going to eat MUCH more cpu than it needs to. You have a while loop running with no wait. This means LabVIEW will use most of the CPU to poll the one control and constantly resize the activeX container. Much better to use an Event structure to look for the control to change.
