You cannot open Volume Mixer when you use a tablet pen on a Windows Vista Tablet PC
On a computer that is running Windows Vista Tablet PC, you use a tablet pen to click the Sound icon in the notification area. However, when you click Mixer, Volume Mixer does not open. In this scenario, you must click Mixer several times to open Volume Mixer.
This problem also occurs when you use a mouse to open Volume Mixer.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB973021.