Windows 7 - Open the Volume Control popup from the command prompt

sndvol.exe is what you want, you just need to use the -f flag to make it show just the master volume slider:

sndvol.exe -f

Running just sndvol.exe opens the volume mixer, as you noted.

sndvol.exe allows you to specify window location by adding coordinates after -f switch:

sndvol.exe -f 26214900

This example creates window at x=500 y=400

coordinates = y * 65536 + x, where x and y - signed integers

BTW there is third command line parameter... may be handle?...

Addendum:

As mentioned in my comment, using sndvol -f as per @Indrek's answer opens the Volume Control window in an unusual location, away from the system tray. Using AutoHotkey, we can make the popup appear where we see fit:

SetWinDelay, -1
Run, % "sndvol -f"
WinWaitActive, ahk_class #32770
WinMove, % A_ScreenWidth - 84, % A_ScreenHeight - 305 - 48

This script places the window at the bottom right of the screen. You can customize it to by changing the last line (WinMove).