Allow the recording of only one program's output.

Changing the default sound card is nice and all, but everything is going through there. For example if you're doing other stuff while recording a radio and there's a system sound it gets recorded as well.

This may actually not be possible, unless you can identify the source of the sound. There use to be possible in older versions of Windows using a user-mode virtual driver but as far as I know that's not an option nowadays.