What other application is using my sound card?

First off, if the answer to this is simply "you can't," I'm okay with that.

That said - in Windows 7, I open the properties of the audio out on my sound card, change the default format from 16 bit to 24 bit (or vice versa) and get this message:

... what is "another application?" Is there any way to find something, anything - the name of the file, the path to the .exe or whatever - anything that will tell me what application has a stranglehold on my sound card?

If it helps, my sound card is an external, M-Audio fast track pro, and yes, the drivers are current.

The Volume Control application, accessible by right-clicking on the volume icon in the notification area next to your clock on the taskbar, or by running sndvol, should identify what applications are presently using your sound card.