Per-application audio control that works in Windows 7 64 Bit

Volumouse seems to do what you want. But you will have to set it up.

Main Window of Volumouse

In my example setup, I click the More button of first slot and choose "Change the volume for application in focus" under Windows 7/Vista/2008 Options.

More options window for first slot

I can now hold Alt key and roll my mouse wheel to control volume of the window (focused) window. It works in Firefox, Opera, Steam and itself but fails in Chrome (probably because of the multiple processes). So I use slot 2 specifically for Chrome and I click the More button to set it up like this:

More options window for second slot

I also chose to use hot-keys (keyboard shortcuts) instead of mouse wheel to showcase that feature here. I can now control Chrome's volume using the keyboard shortcuts anywhere (no need to switch to Chrome first), another advantage of exe-specific volume control. Note that you don't need to specify full path, but it's safer to do so. You can also just specify the file name (chrome.exe).

Using Volumouse with keyboard shortcuts: Combining the settings "Use hot-keys instead of mouse wheel" (blue method) and "Change the volume for application in focus" (red method) worked better for Psycogeek as the hot-keys were functioning in applications where the mouse wheel didn't.

Another tip: If you want an easy way to bring up the standard Volume Mixer of Windows, you can create a shortcut to C:\Windows\System32\SndVol.exe and assign a keyboard shortcut in the shortcut's properties. Or you can pin it on the taskbar.

Another thing to note: When using a 64Bit system, volumouse is running 2 processes a 32bit and a 64bit. If your having problems with it controlling only 32bit or only 64bit programs, check to see if both 32&64 were installed and running.

As far as I know CheVolume is new to the scene and the only application as of this post that can change the audio output device per application.

There is a lengthy thread that began in 2009 of users seeking this ability, so it's been quite the ongoing request. The developer himself answered the thread a couple months ago.

I do not know if CheVolume supports hotkeys.

The website also supplies a contact form. (The developer also availed himself to feedback in this old reddit discussion.)

I'm lifting the FAQ from the website:

How does it work ?

CheVolume gives you power over your applications. You can transfer an open application from your headset to your speakers in a click (or any other audio device, USB, HDMI, etc.).

Can i have 2 different applications on 2 different audio devices?

Hmmmm... YES ! This is the main goal of CheVolume !

Does CheVolume support web-browsers ?

Currently, only Chrome is supported. Firefox and Internet Explorer will be supported on a next update.

Does it work with my games?

Yes ! But some games need to be restarted (or mute/unmute) so the transfer can take place.

If I have a dual-screen, can i have a game on one screen in my headset, and a movie on the other screen, with the sound on the speakers so my girlfriend can watch it?

Yes ;)