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