Seems Microsoft started hiding the Sound Mixer with the Windows 7 release or sometime there 'bouts.� As far as I know all variations of Windows have it but may not be enabled to see.� If you have a speaker icon in the lower right quick access bar you can right-click it with the mouse then select it.
I found this web page to be helpful http://www.wintuts.com/Volume-Mixer-icon-on-taskbar
The volume mixer is helpful to set or mute individual device levels and see what audio devices are being used.

With a virtual audio cable VAC or VB-Cable (that I use) you can be running your favorite decoder software and turn the speaker volume down or mute it without affecting the software decoding.� The DSDPlus software I have been playing with lately is nice because it has the option to use the pc's local IP so no additional software is needed to install.� With digital trunking two receivers are needed and easily configured using a second IP,

73,
Dale, WB8CJW