how to determine what application is playing alert sound?

I am attempting to solve two issues with application notification sounds on my Yosemite running MacBook Pro Early 2011.

First, I expect that setting the OS to DND should mute all application alert sounds. Mine seems to mute things like calendar but does not mute Skype. I've tried resetting PRAM but it made no difference.

Second, there are applications playing notifications during the day that I cannot place. I expected in the Notification Control Panel that if I sort the list by 'Time' it should list them in order of their last playing a notification sound. FWIW all of my apps in that CP are in the Notification Center rather than outside.

My expectation is that the Notification Center decides when an application is allowed to play an alert sound. But I cannot seem to successfully manage things from there. Is the described behavior by design and I need to track down things running in the background or is my OS not behaving as designed? FWIW this has been going on for an indeterminate time, just my family's nerves are starting to fray at the noises-especially in the middle of the night.

Thanks.