Kind of a dead post but I just wanted to mention that the program nircmd.exe has a function
setappvolume
. For example:
nircmd setappvolume plexamp.exe .2
That will set plexamp.exe to have 20% of the master volume level. This will also be reflected in the volume mixer.
Here's their blog announcing it
Here's the download (scroll to the bottom). I always have nircmd.exe in my system32 folder.