I'm found no way via command prompt for some silly reason... My current solution of a small c# program pinvoking SendMessage (in user32.dll) I feel isn't quite the best way of going about making a shortcut to mute the volume on vista, but I have yet to find a simple solution.
According to this post:
Using VBscript you can control the master volume Mute with the following:
Set WshShell = CreateObject("WScript.Shell") WshShell.SendKeys(chr(173))
or (using Hex values)
Set WshShell = CreateObject("WScript.Shell") WshShell.SendKeys(chr(&hAD))
This sends the keycode value used by multimedia keyboards and is a toggle.
Just save it as a .vbs file and set up a shortcut to that file.