As Bilal mentioned in his previous post you will need to use the Windows multimedia SDK functions for this. In particular I would take a look at the waveOutSetVolume function. I have posted a link that gives a little more information on this function. There are some examples, but they are visual basic rather than ANSI C. However it should help to give a general idea of how to use the function. Hope this helps!
Link to help on waveOutSetVolumeMessage Edited by Patrick P. on 02-05-2007 07:22 PM
Pat P.
Software Engineer
National Instruments