Which window is playing that ad sound — and why can't I find it?

This one is almost always a browser tab or a minimized app, not something sitting visibly on the taskbar. Next time you hear it, right-click the speaker icon and open Volume Mixer — it lists every app actually playing sound, even if its window is hidden or minimized. If you see a browser there, look for a tab with a speaker icon, and also check any other browser windows you might have open on another virtual desktop. If nothing shows up in the mixer, open Task Manager and look for a second browser or media player running in the background. Once you find the culprit, mute that tab or close that app, and if it keeps coming back, check that browser's autoplay and notification settings for the site involved.