I don't think that is sufficient. You must also make the device which will create the ding-dong the Primary Sound Device. Of course this needs the device to be there and active in the first place, so you won't know what you're actually doing and what you're after soon.
But you can try to interpret the test the other way around : if the device is not Primary it will never make sound, because that other Primary is addressed (which is ? ...).
Assuming that now your are a 100% confused, make the mobo's sound device the primary (you know, that one which doesn't have anything connected to it -> HD Audio SPDIF or something). So, now it will be quiet, right ? True true, but this is because there's nothing behind it.
When you have done this, you will be sitting back and start to draw conclusions that trying it all like this does not work. It doesn't tell anything. Still you must appoint the Windows sounds to a device with nothing connected by means of making that device Primary. But the main reason is because you'd otherwise hear the ding-dong through your speakers. Not when you are playing music (because the device is taken exclusively with both Engine#3 and #4) but because it may be way loud when no preamp is used (this by itself is because you have all the Windows volumes at 100% by nature haha).
Lastly, if you remove the device in concern as much as possible this will help theoretically and Engine#4 will still be able to use it. But Engine#3 will not (it won't be there anymore for selection and/or useage by WASAPI).
All 'n all you are hunting something which can't exist, which is also why I have huge difficulties putting itinto words. Maybe others can do better here. And remember, this is not XP where -whatever it is you are hunting for- really could help for DirectSound (make it bit perfect). With Vista and up this won't help anyway. Guaranteed !
But it is not necessary either for Engine#3 and #4.
Peter
PS: Don't forget the USB ding-dong hint, because other blings or boings etc. are not there anyway once "windows sounds" have been shut off.