The best suggestion that I can offer is to search for the Windows API that will adjust volume (I'm not familiar enough with Windows to tell you that) and then you should be able to pick the appropriate interface for LabVIEW to use. Best places to look for the Windows infomation would be Microsoft's website or Google. If you can find out what Windows API will adjust the volume, post what you find out, and I'd be happy to help you with the LabVIEW side of things.
Kind Regards,
E. Sulzer
Applications Engineer
National Instruments
