Running the retail/release version of Windows 10, every time I got a UAC prompt or 'toast' notification, the notification sound would be at full volume. I'd go into the Volume Mixer, run it down to around 5%, and sure enough, the next notification ding would be ear-piercing again.
I've found one other person reporting the same bug on a pre-release version.
I've found that leaving the Volume Mixer window open prevents the issue from occurring. Then again, after a few test cycles, I stopped getting ANY System Sounds at all...
I use a Creative USB Sound Blaster HD and have all other audio output devices disabled. I don't know if this affects the issue.