You should be able to access (and modify) the Windows audio settings such as mute and master volume via the winmm.dll library. In LabVIEW, you can use a Call Library Function Node located on the functions palette under Connectivity >> Libraries and Executables >> Call Library Function Node to call this dll.
The following forum post seems to deal with a similar issue; some of the ideas on this post apply to your question as well. There is also some example code posted by another LabVIEW user that may get your started.
http://forums.ni.com/ni/board/message?board.id=170&message.id=22278&requireLogin=False
In addition, the following Developer Zone article on ni.com gives an example of retrieving the current volume level by calling winmm.dll:
It may be helpful for you to have a reference of the function calls available in winmm.dll; you can access this information on the Microsoft MSDN website under:
I hope this helps!
Casey Weltzin
Applications Engineer
National Instruments
