From what I understand there are API calls, but I'm guessing only the program is able to change it's own volume.
http://blog.nirsoft.net/2011/02/25/how-to-control...indows-72008vista/
Your best bet would be to make a simple VB program that detects hotkeys and get it to call "NirCmd.exe .... " from there. That way there's no clicking on shortcuts or whatever and you can have the hotkeys. You could even make it so, for example, the VB program stores the volume for a certain process and increases it by x when pressing F6, or down by x for F5
EDIT: The program will actually let you change the volume by x amount instead of forcing you to set an absolute value