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

There is a Microsoft Store app available that will provide the information at a click. 

https://apps.microsoft.com/detail/xpdfktw721hph2?hl=en-US&gl=US

Start with Volume Mixer while the ad is playing. Press Win+R, type sndvol, and press Enter. Watch which app’s audio meter moves, then mute it briefly to confirm the source.

If it’s a browser, check for a speaker icon on its tabs, including tabs in other browser windows. Muting the suspected tab will help confirm it. A background tab can keep playing audio even when its window is minimised.

If the browser keeps playing unexpected ads, review its extensions and website notification permissions. If the mixer identifies an unfamiliar app instead, check it under Installed apps and run a Windows Security scan.

The sound doesn’t necessarily come from a visible window: background apps and browser tabs can play it without a separate taskbar button.

There is an app in the Microsoft store that will tell you all the info on every sound.

There is an app in the Microsoft store that will tell you evert sound and link it.

You are hearing advertising audio without an obvious playing window, so identify the application before assuming anything is hidden maliciously. While the sound is playing, open Settings, System, Sound, Volume mixer. Mute the listed applications one at a time, restoring each unrelated one, until the sound stops. This isolates the application rather than the individual browser tab. If the browser is responsible, inspect its open windows and tabs, including minimized windows; in Edge, Shift+Esc opens Browser Task Manager to help identify running tabs and extensions. Close suspect tabs individually and test again. If the sound returns after all browser windows are closed, note which application appears in Volume mixer at that moment. For a source you cannot explain, update Microsoft Defender’s security intelligence and run a scan through Windows Security. The audio alone is insufficient evidence of an infection.

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.